Intraperitoneal Route of Drug Administration: Should it Be Used in Experimental Animal Studies?

Abstract

Intraperitoneal (IP) route of drug administration in laboratory animals is a common practice in many in vivo studies of disease models. While this route is an easy to master, quick, suitable for chronic treatments and with low impact of stress on laboratory rodents, there is a common concern that it may not be an acceptable route for drug administration in experimental studies. The latter is likely due to sparsity of information regarding pharmacokinetics of pharmacological agents and the mechanisms through which agents get systemic exposure after IP administration. In this review, we summarize the main mechanisms involved in bioavailability of IP administered drugs and provide examples of pharmacokinetic profiles for small and large molecules in comparison to other routes of administration. We conclude with a notion that IP administration of drugs in experimental studies involving rodents is a justifiable route for pharmacological and proof-of-concept studies where the goal is to evaluate the effect(s) of target engagement rather than properties of a drug formulation and/or its pharmacokinetics for clinical translation.

Full text

EXPERT REVIEW
Intraperitoneal Route of Drug Administration: Should it Be Used
in Experimental Animal Studies?
Abdullah Al Shoyaib
1
&Sabrina Rahman Archie
2
&Vardan T. Karamyan
1,3
Received: 8 January 2019 /Accepted: 27 November 2019
#Springer Science+Business Media, LLC, part of Springer Nature 2019
ABSTRACT Intraperitoneal (IP) route of drug administra-
tion in laboratory animals is a common practice in many
in vivo studies of disease models. While this route is an easy
to master, quick, suitable for chronic treatments and with low
impact of stress on laboratory rodents, there is a common
concern that it may not be an acceptable route for drug ad-
ministration in experimental studies. The latter is likely due to
sparsity of information regarding pharmacokinetics of phar-
macological agents and the mechanisms through which agents
get systemic exposure after IP administration. In this review,
we summarize the main mechanisms involved in bioavailabil-
ity of IP administered drugs and provide examples of phar-
macokinetic profiles for small and large molecules in compar-
ison to other routes of administration. We conclude with a
notion that IP administration of drugs in experimental studies
involving rodents is a justifiable route for pharmacological and
proof-of-concept studies where the goal is to evaluate the
effect(s) of target engagement rather than properties of a drug
formulation and/or its pharmacokinetics for clinical
translation.
KEY WORDS biopharmaceutical .intraperitoneal .
macromolecule .pharmacokinetics .small molecule
INTRODUCTION
It is well-recognized that the route of administration is a crit-
ical determinant of the final pharmacokinetics, pharmacody-
namics as well as toxicity of pharmacological agents (1).
Intravenous (IV), subcutaneous (SC), intraperitoneal (IP) and
oral routes are the main paths of drug administration in lab-
oratory animals, with each offering advantages and disadvan-
tages depending on specific goal(s) of the study. One of the
more commonly used routes in rodent studies is the IP route
where a pharmacological agent is injected into peritoneal cav-
ity. This easy to master technique is quick and minimally
stressful for animals. It involves holding of the rodent in a
supine position with its head tilted lower than the posterior
part of the body and insertion of the needle in the lower
quadrant of the abdomen (at ~10° angle) with care to avoid
accidental penetration of the viscera (2–4). Large volumes of
solution (up to 10 ml/kg) can be safely administered to rodents
through this route (5) which may be advantageous for agents
with poor solubility. This route is especially common in chron-
ic studies involving mice for which repetitive IV access is chal-
lenging. In most cases, IP administration is also preferred over
the oral route for biological agents to avoid the GI tract and
potential degradation/modification of biopharmaceuticals.
The main disadvantage of this route is that it is minimally
used in clinic (mostly for treatment of peritoneal cancers),
because of which its use in experimental studies is often ques-
tioned and discouraged. To mitigate this concern, in this re-
view article we discuss the anatomy and physiology of the
peritoneal cavity, and the mechanisms governing absorption
of substances from peritoneal cavity. In addition, we provide
examples and compare pharmacokinetic profiles of small and
large molecules upon IP and other routes of administration in
experimental animals. Based on the discussed experimental
evidence, we conclude that IP administration of drugs in ex-
perimental animals is a justifiable route for pharmacological
and proof-of-concept studies where the goal is to evaluate the
*Vardan T. Karamyan
vardan.karamyan@ttuhsc.edu
1
Department of Pharmaceutical Sciences, School of Pharmacy
TTUHSC, Amarillo, Texas, USA
2
Department of Pharmaceutical Technology, Faculty of Pharmacy
University of Dhaka, Dhaka, Bangladesh
3
Center for Blood Brain Barrier Research, School of Pharmacy
TTUHSC, 1300 Coulter St., Amarillo, Texas 79106, USA
Pharm Res (2020) 37: 12
https://doi.org/10.1007/s11095-019-2745-x
/ Published online: 23 December 2019

effect(s) of target engagement rather than properties of a drug
formulation and/or its pharmacokinetics for clinical
translation.
ANATOMY AND PHYSIOLOGY
OF PERITONEAL CAVITY
Peritoneal cavity is a closed space within the abdomen
that contains the abdominal organs and is derived from
the coelomic cavity of the embryo. Peritoneal cavity is
lined by the most extensive serous membrane in the body
(i.e., peritoneum) which has total surface area equaling to
that of the skin surface (6). Peritoneal cavity is filled with a
thin film of fluid (peritoneal fluid) comprised of water,
electrolytes, proteins, cells and other substances originat-
ing from the interstitial fluid of the adjacent tissues. In
humans, the volume of peritoneal fluid ranges from 50
to 75 ml (7), whereas in mice its volume ranges between
0.02 and 0.1 ml (8). In addition, the peritoneal fluid con-
tains leukocytes and antibodies to fight off infections, and
plasma proteins at concentration that is about 50% of
whatisfoundinplasma(9).
Peritoneum covers most of the intra-abdomenal organs
and consists of a single layer of squamous mesothelial cells
(10). The mesothelial cell layer sits on a thin basement mem-
brane and the majority of these mesothelial cells are flattered
type with an approximate diameter of 25 μm. Mesothelial
cells are closely connected to each other by either tight junc-
tions, adherens junctions, gap junctions or desmosomes (11).
The sub-mesothelial layer of peritoneum contains collagen,
adipose tissue, lymphocytes, blood vessels as well as lymphatics
(12). Fibroblasts and occasional macrophages are also present
in this part of the peritoneum (13). Notably, the apical surface
of mesothelial cells contain microvilli of different length, shape
and density, which increase the functional surface area of the
peritoneum (Fig. 1)(14).
Peritoneal mesothelial cells play crucial role in mainte-
nance of peritoneal homeostasis and transport of fluids and
solutes across the membrane. Intra-abdominal organs and
mesentery are supported by the visceral peritoneum, whereas
the parietal peritoneum lines up the abdominal wall, pelvis,
anterior surfaces of retroperitoneal organs, and inferior sur-
face of the diaphragm. The peritoneum minimizes friction
and facilitates free movement between abdominal viscera,
resists or localizes infection, and stores fat, especially in the
greater omentum (15,16). Importantly, the peritoneal meso-
thelial cells possess many features of epithelial cells including
presence of polygonal, cobblestone morphology with surface
microvilli and an ability to form a polarized monolayer that
permits unidirectional transport of molecules (17). The peri-
toneal fluid has a pH of 7.5–8.0 and buffering capacity (18),
because of which substances are rarely ionized in the perito-
neal cavity after IP administration (19).
PERITONEAL BLOOD FLOW
The sub-mesothelial layer of the peritoneum supports a com-
plex but efficient vascular network of blood and lymphatic
vessels (20,21). Most of the circulatory vessels are blood capil-
laries but few arterioles and venules are also present in the sub-
mesothelium (22). The density of these sub-mesothelial micro-
vessels varies along different portions of the peritoneal cavity.
For example, in the rabbit, the most vascularized area of peri-
toneal cavity is mesentery, which contains about 71% of peri-
toneal microvessels, whereas the diaphragm and parietal peri-
toneum contain about 18% and 11% of microvessels, respec-
tively (21,23). Through these capillaries, peritoneal cavity
receives 4 to 7% of the total cardiac output in the rabbit (24)
and rat (25,26). The average peritoneal blood flow rate in the
rat is 2.5–6.2 ml/min/kg (24). Notably, blood supply to vis-
ceral peritoneum and intra-peritoneal organs originates from
the coeliac, superior and inferior mesenteric arteries, whereas
the parietal peritoneum is irrigated form the branches of cir-
cumflex, iliac, lumbar, intercostal and epigastric arteries
(27,28). Similarly, the venous system that drains from the vis-
ceral peritoneum empties into portal vein, whereas the veins
that drain from the parietal peritoneum empty into the infe-
rior vena cava (28,29). Overall the entire peritoneum is well
perfused with blood capillaries and provides an excellent sur-
face for exchange of drugs between the peritoneal cavity and
plasma (24).
PERITONEAL LYMPHATIC SYSTEM
Like many other organs, peritoneum has a well-structured
lymphatic network which maintains the solute and fluid bal-
ance of peritoneal tissues and prevents formation of edema
(30,31). In the sub-mesothelial layer of the peritoneum, the
terminal lymphatic capillaries are subdivided into parasternal,
paravertebral, mediastinal, intercostal and retroperitoneal
lymphatics. Parasternal, mediastinal and retroperitoneal lym-
phatics are arranged over the anterior, posterior and central
part of the diaphragm, respectively. These terminal lymph
capillaries unite together to form afferent collecting and pre-
nodal lymphatic ducts which are then connected with regional
lymph nodes. The parasternal, paravertebral and mediastinal
lymphatics are connected with the mediastinal lymph nodes,
whereas, retroperitoneal lymphatics are joined with cisternal
and intestinal lymph nodes. From these lymph nodes the lym-
phatic system joins the venous circulatory system through tho-
racic duct and right lymphatic duct (32).
12 Page 2 of 17 Pharm Res (2020) 37: 12

ABSORPTION OF SOLUTES
FROM THE PERITONEAL CAVITY, GENERAL
CONSIDERATIONS
Peritoneal cavity is an excellent portal of entry into systemic
circulation for substances after IP administration. Its large
surface area, estimated about 125 cm
2
in an average rat, pres-
ence of microvilli on mesothelial cells and the vast blood sup-
ply facilitate rapid absorption of substances after IP adminis-
tration (33,34). Additionally, lymphatic transport of substrates
significantly contributes to the total transfer from the perito-
neal cavity to systemic circulation (35).
To reach the vascular compartment after IP administra-
tion, a compound must cross different structures in and
around the peritoneal cavity including the peritoneal fluid,
mesothelium, sub-mesothelium and blood vessel wall (36,37).
Transcellular and intercellular spaces in the visceral and pari-
etal mesothelium are the main gateways for solutes and mol-
ecules to pass from the peritoneal cavity into surrounding
tissues. Because of its structural features, visceral mesothelium
is more permeable to molecules compared to parietal meso-
thelium. Anatomically, visceral peritoneum is composed of flat
cells with large number of pinocytic vesicles which facilitate
absorption of molecules. On the contrary, parietal peritone-
um contains less pinocytic vesicles and has a more developed
basement membrane/connective tissue which make it less per-
meable for molecules (Fig. 1)(38).
Experimental studies indicate that small to medium size
molecules (MW < 5000) and fluids are predominantly
absorbed from visceral peritoneum by diffusion through the
splenic, inferior and superior mesenteric capillaries and drain
into the portal vein (39). On the other hand, large molecules
(MW > 5000), proteins, blood and immune cells are taken up
by the lymphatics (Fig. 2)(40). It is noteworthy, that there is
some amount of retrograde movement, of both small and
large molecules, from capillaries to peritoneal cavity, however
it minimally affects the overall absorption of pharmacological
agents from peritoneal cavity to systemic circulation (41,42).
The physiological mechanisms of fluid and solute move-
ment from peritoneal cavity-to-blood or blood-to-peritoneal
cavity are the same, either through diffusion or convection.
Rapid absorption of small molecules from the peritoneal cav-
ity into blood capillaries are mainly governed by effective sur-
face area (A), solute concentration gradient (ΔC) and solute
permeability (Ps) (42,43).
Mass transfer of solute JsðÞ¼A*ΔC*Ps
In case of IP administration, the effective surface area (A) is
created by the microvilli on mesothelial cells, which ensure
increased absorption of IP administered substances. Because
substances are efficiently carried away by capillary blood flow,
a constant concentration gradient (ΔC) is available between
the absorption site of the peritoneal cavity and surrounding
capillaries. This concentration gradient facilitates the diffusion
of administered substances from peritoneal cavity into blood
capillaries. Lastly, physicochemical properties of the adminis-
tered substance determine its permeability (Ps) and hence ab-
sorption from peritoneal cavity. A highly lipophilic substance
has a higher distribution rate into tissues, which results in
Fig. 1 Panel (a), parietal
mesothelial cell with small number
of pinocytic vesicles and a more
mature basement membrane.
Panel (b), visceral mesothelial cell
with higher number of pinocytic
vesicles and less mature basement
membrane.
Pharm Res (2020) 37: 12 Page 3 of 17 12

rapid removal of substance from the systemic circulation. The
latter increases the concentration gradient between blood cap-
illaries and peritoneal cavity, and leads to increased absorp-
tion rate of the substance after IP administration (44).
Notably, solute permeability is the main determinant of con-
vection rate, whereas concentration gradient is the main driv-
ing force for diffusion (45).
CAPILLARY ABSORPTION OF SOLUTES
AFTER IP ADMINISTRATION
In the peritoneal microvascular network, majority of the sol-
ute and fluid exchange between peritoneal cavity and circula-
tory system occurs through capillaries. The capillary walls of
the peritoneal capillaries are ‘continuous’type, lined with sin-
gle layer of continuous endothelial cells and basal lamina.
These endothelial cells are very thin (0.5 μm) and highly per-
meable (46,47), plus they contain a large number of cytoplas-
mic vesicles (48–50). Intracellular clefts (6–7nmthinchannels
located between adjacent endothelial cells) are also present in
these capillaries which ensure rapid passage of water soluble
ions and small molecules (21). Absorption of molecules with
molecular size of <20,000 Da from peritoneal cavity occurs by
either diffusion or convection through these capillaries (42,51).
The rate and extent of diffusion depends on the size, charge,
configuration and concentration gradient of the molecules
(52). In addition, IP administration of a drug in solution
increases the IP hydrostatic pressure which drives the convec-
tion of the soluble drugs along with fluid through peritoneum
into the blood capillaries (45). Molecules absorbed from the
visceral peritoneum, mesentery and omentum are drained
into the portal vein, while molecules absorbed form the pari-
etal peritoneal blood capillaries and lymphatics drains directly
into the systemic circulation (53,54). Substances entering
through portal circulation merge with systemic circulation af-
ter passing through liver which results in fast pass metabolism
of the administered substances.
In one extensive experimental study carried out in rats,
Lukas and colleagues have shown that after IP administration
of small molecules (atropine, caffeine, glucose, glycine and
progesterone), the primary route of absorption is through por-
tal circulation (29). In this study the small molecules were
selected to represent different physicochemical properties (ba-
sic, nonionic hydrophilic, zwitterionic and nonionic hydro-
phobic) and it was revealed that they all had comparable
absorption pattern. In addition, the rate of absorption be-
tween IP and sub-cutaneous (SC) administrations of these
Fig. 2 Schematic overview of
absorption pathways for small and
macromolecules from the
peritoneal cavity to systemic
circulation.
12 Page 4 of 17 Pharm Res (2020) 37: 12

compounds was compared. IP administered compounds
could be detected in systemic circulation as soon as after 10 s
of administration whereas it took about 60 s for SC adminis-
tered compounds to reach systemic circulation. Another no-
table distinction between these two routs was the substantially
lower degree of liver exposure and hence less biotransforma-
tion of molecules administered SC (29). Additionally, lipid
solubility of IP administered compounds can affect the rate
of their absorption. In general, increased lipid solubility leads
to increased absorption from peritoneal cavity; e.g. only
~57.4% of barbital (lipid-water partition co-efficient of
0.001) was shown to be absorbed after IP administration in
the rat, whereas absorption of thiopental (lipid-water partition
co-efficient of 3.3) was shown to be 96.1% in the same study
(55). Another important factor in absorption of IP-
administered compounds is their ionization state at physiolog-
ic pH, which can be affected by buffering capacity of the
peritoneal cavity (55). In general, absorption of IP-
administered acidic substances increases as their pKa
increases, e.g. benzyl penicillin with pKa of 2.8 has an absorp-
tion of 16.5%, whereas secobarbital with pKa of 7.9 has an
absorption of 87% (55). On the other hand, absorption of IP-
administered basic substances decreases as their pKa
increases, e.g. caffeine with pKa of 0.9 has an absorption of
68.3%, whereas atropine with pKa of 9.6 has an absorption of
27% (55). Overall, unionized compounds are absorbed to a
greater extent after IP administration than ionized com-
pounds. Importantly, viscosity of the IP-administered formu-
lations can also affect the absorption of pharmacological
agents, with higher viscosity leading to decreased absorption
and efficacy (55).
LYMPHATIC ABSORPTION OF SOLUTES
AFTER IP ADMINISTRATION
Cellular arrangement around the peritoneum and ample sup-
ply of blood and lymph vessels assures rapid absorption of not
only small molecules but also proteins and cells after IP ad-
ministration (56,57). Molecules with molecular weight larger
than 30,000 Da enter the systemic circulation from the peri-
toneal cavity primarily via lymphatic vessels (41).
Experimental studies indicate that IP administered plasma
proteins are completely absorbed into systemic circulation
and are distributed throughout the body similar to IV admin-
istered plasma proteins (58). Following IP administration, pro-
teins are absorbed by lymphatic vessels, which are especially
enriched around the diaphragm in most mammals including
mouse, rat, dog, cat and human (59,60). In addition to pro-
teins, small particles and cells are also absorbed mainly
through end lymphatic vessels, which are also known as sto-
mata (61,62). The peritoneal stomata are physiological open-
ings in the peritoneum. Here, the mesothelial cells are
interrupted and share a common basement membrane with
lymphatic endothelium which allows a direct communication
between peritoneal cavity and underlying lymphatics (11).
Notably, relaxation of the diaphragm during respiration also
controls the absorption of macromolecules from the peritone-
al cavity (63). The diaphragm relaxes during expiration and
the adjacent mesothelial cells on the border of lacunae sepa-
rate from each other creating suction force and facilitating
absorption of macromolecules. However, contraction during
inspiration results in closing of gaps between mesothelial cells
and emptying of lacunae into the efferent lymphatics. The
parasternal lymphatics located in the diaphragm, especially
in the right half around the liver, are mainly responsible for
the transfer of peritoneal deposits form peritoneal cavity into
the mediastinal nodes and then into venous circulation
through the right lymphatic duct (32,62). A small portion of
peritoneal deposits are also drained through the paravertebral
and mediastinal lymphatics situated on the parietal peritone-
um, mesenteric, and omentum (60). The visceral lymphatics
collect solutes and fluids from the omentum as well as mesen-
tery and drain into the complex network of visceral lymph
nodes. Subsequently, these visceral lymphatics collectively
drain into parietal lymph nodes and finally dump the contents
into venous circulation primarily through the thoracic duct
(64). Quantitatively, about 75% of the absorbed proteins are
drained into the right lymph duct and about 25% into the
thoracic duct, which subsequently merge with venous circula-
tion (Fig. 3)(57,65). Notably, obstruction of the right lymphat-
ic and thoracic ducts does not prevent systemic absorption of
proteins from the peritoneal cavity because trace amounts of
proteins can still be absorbed through other small lympho-
venous communications and capillary walls (60).
PHARMACOKINETICS OF SMALL MOLECULES
ADMINISTERED VIA IP VS OTHER ROUTES
Recently, Durk and colleagues carried out a comprehensive
study in rats to compare the pharmacokinetic parameters of
IP and SC administered 9 small molecules (carbamazepine,
citalopram, desmethylclozapine, diphenhydramine, gabapen-
tine, metaclopramide, naltrexone, quinidine and risperidone)
with distinct physicochemical properties (66). For all mole-
cules, administered at 1 mg/kg dose, IP administration
yielded higher C
max
and lower t
max
values in comparison to
the SC route (Table I). The authors also compared AUC
0–360
ratio of the brain interstitial fluid and plasma for all 9 mole-
cules after IV infusion (1 mg/kg/h for 6 h) or 3 intermittent
doses of IP or SC injections (2 mg/kg, every 2 h) (66–68).
These experiments revealed that after IP administration, the
brain to plasma ratios were greater than 1 for diphenhydra-
mine and naltrexone, close to 1 for carbamazepine, metaclo-
pramide and risperidone, and substantially less than 1 for
Pharm Res (2020) 37: 12 Page 5 of 17 12

citalopram, desmethylclozapine, gabapentin and quinidine.
Notably, brain exposure of most compounds were higher after
IP or SC administration in comparison to IV infusion which
could be due to saturation of transporters at the blood brain
barrier and rapid elimination rate after IV infusion (Table II).
Several other studies also compared pharmacokinetic
properties of small molecular drugs administered via IP and
other routs (Table III). Among them, a study carried out by
Shimada and colleagues compared plasma and peritoneal
concentrations of docetaxel (8 mg/kg, MW = 807.89) after
IP and IV administration in mice (69). In intact mice, AUC
of 4.85 and 3.37 mg h/ml were observed after IV and IP
administration of the drug, respectively. Based on these data
the calculated absolute bioavailability (F% = AUC IP/AUC
IV × 100%) of IP administered docetaxel is 69% (69),
which is much higher compared to the absolute bio-
availability of docetaxel after oral administration
(~2.8%) (70). Notably, the documented maximum plas-
ma concentration (C
max
) of docetaxel was also similar
after IP and IV administration in this study.
In another study, the plasma profile of deramciclane
(10 mg/kg, MW = 301.466) was compared after oral, IP and
IV administration in rats (71). Deramciclane showed AUC
0-∞
of 106.95, 578.18 and 3127.53 ng-h/ml after oral, IP and IV
administration, respectively. The absolute bioavailability of
the drug was almost 6-fold higher after IP vs oral administra-
tion. Furthermore, the time required to reach maximum plas-
ma concentration of the drug was 4 times shorter after IP vs
oral administration. Importantly, in case of both oral and IP
routes of administration deramciclane had poor bioavailabil-
ity due to heavy first pass metabolism in the liver.
One other study compared pharmacokinetic profile of IP
vs oral or IV administered
18
F-fluorodeoxyglucose (FDG,
MW = 181.15 g/mol) for positron emission tomography/
computed tomography (PET/CT) in mice (72). The authors
concluded that the AUC and tissue uptake of FDG was great-
er in case of IP administration compared to the oral route.
The time to reach maximum tissue concentration (t
max
)was
shorter after IP administration indicating a more rapid ab-
sorption of FDG after IP vs oral administration. Notably,
the brain accumulation of FDG was not significantly different
after IP or IV administration, however, it was lower after oral
administration (72).
In another recent study, Matzneller and colleagues com-
pared bioavailability of tariquidar (MW = 646.73 g/mol) at
15 mg/kg dose in two different formulations after oral, IP and
IV administration in rats (73). For formulation A (tariquidar
dissolved in 5% glucose and 2% DMSO solution), the AUC
were 18.1 μg.h/ml, 23.8 μg.h/ml and 25.2 μg.h/ml after oral,
IP and IV administration, respectively. Whereas, C
max
were
1.2 μg/ml, 1.5 μg/ml, 1.9 μg/ml and t
max
were 4 h, 2 h, and
0.5 h after oral, IP and IV administration, respectively. These
parameters were further improved in formulation B (micro-
emulsion of tariquidar), for which AUC were 21.9 μg.h/ml,
25.6 μg.h/ml and 25.2 μg.h/ml, C
max
were 1.3 μg/ml,
1.6 μg/ml, 1.9 μg/ml and t
max
were 3.6 h, 2 h, and 0.5 h after
oral, IP and IV administration, respectively. Notably, for both
formulations, the absolute bioavailability (F%) was higher af-
ter IP vs oral administration; for formulation A, F% was 71.6
and 91.4 after oral and IP administration, for formulation B,
F% was 86.3 and 99.6, respectively.
In a similar study carried out in mice, it was revealed that
IP administered lenalidomide (MW = 259.261 g/mol) has
better bioavailability compared to oral administration (74).
Administration of the drug at 10 mg/kg dose resulted in
AUC of 214 μg.min/ml, 300 μg.min/ml and 284 μg.min/
ml after oral, IP and IV administration, respectively. The
absolute bioavailability (F%) was 75 and 105 after oral and
IP administration, respectively. Whereas, the absorption rate
was higher after IP vs oral administration, with an absorption
constant (K
a
) of 0.044 min
−1
and 0.014 min
−1
, respectively.
In summary, these experimental studies indicate that IP
administration of small molecule pharmacological agents
Fig. 3 Schematic representation of the main lymphatic absorption pathways
for macromolecules after IP administration.
12 Page 6 of 17 Pharm Res (2020) 37: 12

results in faster and more complete absorption compared to
oral and SC routes. Given the fast absorption of most sub-
stances from peritoneal cavity, it is generally considrered that
systemic exposure (AUC and C
max
) of a IP-admnistered sub-
stance is closer to that of the IV route. However, it is difficult
to state how similar the exposure profiles are for the two
routes, expecially considering that IP-administered substances
are subject to first pass metabolism similar to orally adminis-
tered substances (29,75). The uncertaininty is reasoned by a
close examination of the published literature indicating
that a good number of pharmacokinetic studies compar-
ing IP and other routes, did not consider the rapid
absorbtion of substances from peritoneal cavity and did
the first blood sampling 30 min after IP administration
(69,73,76). For rapidly absorbed compounds such experi-
mental design could substantially underestimate systemic
exposure of the pharmacological agent after IP adminis-
tration, and provide an indication of higher first pass me-
tabolism. Unfortunately, analysis of the published studies
does not allow to unequivocally conclude whether the first
pass metabolism of IP-administered small molecules is as
extensive as in case of the oral route, and future studies
specifically designed to answer this question will be need-
ed to clarify the notion.
Ta b l e I Pharmacokinetic
Parameters of 9 Distinct Small
Molecules After IP and SC
Administration in the Rat (Tabulated
from Data Reported in Durk et al.,
2018)
Drug MW* Charge Route C
max
(ng/ml) t
max
(h) t
½
(h)
Carbamazepine 236 Neutral SC 305 ± 132 0.41 ± 0.76 2.87± 2.09
IP 426 ± 191 0.11 ± 0.98 0.74 ± 0.18
Citalopram 324 Basic SC 95.6 ± 36.3 0.50 ± 0 1.17 ± 0.04
IP 106 ± 67.1 0.36 ± 0.55 1.33 ± 0.61
Desmethylclozapine 313 Basic SC 212 ± 62.3 1.17 ± 0 3.87 ± 1.35
IP 268 ± 112 1.01 ± 0 2.83 ± 0.46
Diphenhydramine 255 Basic SC 44.5 ± 21.9 0.50 ± 0.57 0.84 ± 0.33
IP 38.7± 15.5 0.044 ± 0.28 0.90 ± 0.47
Gabapentine 171 Zwitter SC 657 ± 127 1.67 ± 0.57 5.88 ± 2.44
IP 1060± 305 0.833 ± 0.28 3.93 ± 1.15
Metaclopramide 300 Basic SC 49.9 ± 17.2 0.50 ± 0 4.25 ± 6.1
IP 74.8± 43.6 0.04 ± 0 0.61 ± 0.12
Naltrexone 341 Basic SC 105 ± 30.3 0.41 ± 0.14 3.68 ± 5.43
IP 128 ± 26.4 0.19 ± 0.26 1.16 ± 1.30
Quinidine 324 Basic SC 84.7 ± 32.8 0.66 ± 0.28 0.99 ± 0.32
IP 111 ± 65.5 0.11 ± 0.12 0.72 ± 0.24
Risperidone 410 Basic SC 156 ± 68.9 0.66 ± 0.28 3.19 ± 3.35
IP 169 ± 67.5 0.04 ± 0 0.72 ± 0.28
*, molecular weight in g/mol; values are mean ± S.D.
Table II Comparison of Brain to
Plasma AUC of 9 Distinct Small
Molecules After IV, SC and IP
Administration in the Rat (Tabulated
from Data Reported in Durk et al.,
2018)
Drug AUC
0–360, ISF
/AUC
0–360, plasma
IV infusion
(1 mg/kg/h for 6 h)
Subcutaneous
(2 mg/kg every 2 h × 3)
IP
(2 mg/kg every 2 h × 3)
Carbamazepine 0.250 ± 0.07 0.779 ± 0.06 1.31 ± 0.23
Citalopram 0.438 ± 0.13 0.450 ± 0.28 0.467 ± 0.15
Desmethylclozapine 0.0902 ± 0.06 NC 0.098 ± 0.04
Diphenhydramine 2.24 ± 0.43 2.43 ± 0.95 3.38 ± 0.94
Gabapentine 0.0153± 0.01 0.0622 ± 0.01 0.0747 ± 0.02
Metaclopramide 0.0905 ± 0.02 0.646 ± 0.31 0.747 ± 0.13
Naltrexone 0.441 ± 0.16 1.55 ± 0.67 2.78 ± 0.65
Quinidine 0.154
a
NC 0.143 ± 0.05
Risperidone 0.620
a
0.776 ± 0.64 0.793± 0.20
Values are mean ± S.D. NC, not calculated in the original article.
a
, S.D. was not reported in the original article
Pharm Res (2020) 37: 12 Page 7 of 17 12

PHARMACOKINETICS OF MACROMOLECULES
ADMINISTERED VIA IP VS OTHER ROUTES
Numerous in vivo studies conducted in various animal models
of disease have shown biological effect(s) of macromolecules
and recombinant proteins after IP administration indicating
bioavailability of large molecules administered by this route
(77–80). In addition, some studies focused on complete or
partial pharmacokinetic characterization of macromolecules
after IP administration providing more detailed information
about suitability of this route for administration of macromo-
lecules (Table IV).
In an elegant study conducted in mice, Sumbria and col-
leagues investigated plasma and brain availability of IgG-TNF
Decoy Receptor Fusion Protein (MW = 210 kDa) after IV, IP
and SC administration at 0.7, 3 and 10 mg/kg doses (81). The
observed C
max
values decreased in the following order IV >
IP > SC for all three doses. The calculated plasma AUC for
10 mg/kg dose were 10.8 mg.min/mL, 94.521 mg.min/mL
and 17.233 mg.min/mL for IV
0-1h
,IP
0–24h
and SC
0–24h
routes, respectively, indicating a 5.5-fold higher AUC
0–24h
after IP vs SC. Notably, while plasma concentrations of the
noted groups were different, the brain concentrations were
not different between IP and SC injections.
A recent comparative study carried out in neonatal rats
evaluated the plasma and brain pharmacokinetics of recom-
binant human erythropoietin (rEpo, MW = 37 kDa, adminis-
tered IP or SC at 5000 U/kg) (82). The calculated plasma
AUC
0-∞
were 140,331 U.h/L, 117,677 U.h/L and brain
AUC
0–24
were 52.5 U.h/L, 45.2 U.h/L after IP and SC ad-
ministration, respectively. Plasma and brain C
max
were
10,015 U/L and 3.3 U/g after IP administration, and
6224 U/L and 2.8 U/g after SC administration, respectively.
Plasma t
max
was 3 h for IP and 9 h for SC administration.
In another study, Veronese and colleagues studied bio-
availability of intact superoxide dismutase (SOD, MW =
37 kDa; 4000 IU) and monomethoxypoly-ethylene glycol
(MPEG) conjugated SOD (3000 IU) after IV, IP, intramuscu-
lar (IM) and SC administration in the rat (83). In case of
MPEG-conjugated SOD, the calculated AUC were 71, 54
and 29% of the administered dose after IP, IM and SC ad-
ministration (compared to IV route), indicating the following
order of bioavailability IV > IP > IM > SC. The plasma C
max
values were ~300 μg/ml, 100 μg/ml, 70 μg/ml and 62 μg/ml
for IV, IP, IM and SC routes, whereas absorption rates de-
creased in following order IP > IM > SC, and t
max
values
were 10 h, 42 h and 40 h, respectively. In comparison to
AUC of IV administered MPEG-conjugated SOD, the ob-
served AUC of intact SOD after IV, IP, IM and SC admin-
istration were 1.20, 0.87, 0.72 and 0.59% indicating superior
stability of the conjugated SOD over the intact form. The
absorption rate of intact SOD was highest after IP adminis-
tration, followed by IM and SC routes (IP > IM > SC), with
t
max
values of 100 min, 150 min and 150 min, respectively.
In another study, Parker and colleagues performed phar-
macokinetic evaluation of Exendin-4 (a homolog of glucagon-
like peptide-1 (7–36)amide, MW = 4186.63 Da) in rats (84).
The observed AUC values were 172 ± 5 nM.h/ml, 128 ±
25 nM.h/ml and 112 ± 18 nM.h/ml following IV, IP and
SC administration of 50 nmol Exendin-4. It was also revealed
that IP administration results in higher C
max
(35.3 ± 6.1 nM)
compared to SC (28± 4 nM) administration, whereas elimi-
nation was also more rapid after IP (t
1/2
= 157 min) compared
to SC (t
1/2
= 216 min) administration.
Ta b l e I I I Pharmacokinetic Parameters of Various Small Molecules After IP and Other Routes of Administration in Rodents
Drug name MW* Route Dose** Pharmacokinetics Ref.
AUC*** C
max#
t
max##
Docetaxel 807.879 IV 8 4850 1.2 NA (69)
IP 8 3370 1.2 1
Deramciclane 301.466 IV 10 3127.53 2643 NA (71)
IP 10 578.18 177.8 0.166
Oral 10 106.95 44.94 0.5
Tariquidar
(In DMSO)
646.73 IV 15 25,200 1.9 0.5 (73)
IP 15 23,800 1.5 2
Oral 15 18,100 1.2 4
Tariquidar
(Microemulsion)
646.73 IV 15 25,200 1.9 0.5 (73)
IP 15 25,600 1.6 2
Oral 15 21,900 1.3 3.6
Lenalidomide 259.261 IV 10 284,000 158 NA (74)
IP 10 300,000 8.48 NA
Oral 10 214,000 2.03 NA
*, molecular weight in g/mol; **, dose in mg/kg; ***, AUC in ng-h/ml; #, t
max
in h; ##, C
max
in μg/ml; NA, not available
12 Page 8 of 17 Pharm Res (2020) 37: 12

Additionally, pharmacokinetic parameters of radiolabeled
soluble interlukin-1 receptor (IL-1R, MW = 68 kDa) were
studied after IV (340 ng), IP (240 ng) and SC (240 ng) admin-
istration in mice (85). Though the study did not report AUC
values, the observed C
max
was 178 ng/mL, 32 ng/mL and
14 ng/mL, and t
max
was 1 min, 120 min and 240 min after IV,
IP and SC administration of IL-1R, respectively. In the same
study, the authors studied pharmacokinetic properties of sol-
uble interlukin-4 receptor (IL-4R, MW = 140 kDa at 1.1 μg).
ForIL-4RtheobservedC
max
was 1.2 μg/mL, 0.19 μg/mL,
0.21 μg/mL, and t
max
was 2 min, 60 min, 60 min after IV, IP
and SC, respectively.
Table IV Plasma Pharmacokinetic
Parameters of Various
Macromolecules After IV, IP and SC
Routes of Administration in Rodents
1 MW (kDa) Route Dose Pharmacokinetics Ref.
AUC* C
max
*t
max
IgG-TNF 210 IV 10 mg/kg 10.8
(0-1h)
~250 NA (81)
IP 10 mg/kg 94.52
(0–24h)
120 6 h
SC 10 mg/kg 17.23
(0–24h)
20 6 h
rEpo 37 IP 5000 U/kg 140,331 10,015 3 h (82)
SC 5000 U/kg 117,677 6224 9 h
SOD 37 IV 4000 IU 1.20%
a
300 NA (83)
IP 4000 IU 0.87%
a
40 1.67 h
IM 4000 IU 0.72%
a
15 2.5 h
SC 4000 IU 0.59%
a
18 2.5 h
MPEG-SOD 42 IV 3000 IU 100% 300 NA (83)
IP 3000 IU 71% 100 10 h
IM 3000 IU 54% 70 42 h
SC 3000 IU 29% 62 40 h
Exedin-4 4.186 IV 50 nM 172 NA NA (84)
IP 50 nM 128 35.3 1 h
SC 50 nM 112 28 0.5 h
sIL-1R 68 IV 340 ng NA 178 1 min (85)
IP 240 ng NA 32 2 h
SC 240 ng NA 14 4 h
sIL-4R 140 IV 1.1 μgNA 1.22min(85)
IP 1.1 μgNA 0.191h
SC 1.1 μgNA 0.211h
AD-114-PA600-6H 60.6 IV 10 mg/kg 1871 467 1.8 min (88)
IP 10 mg/kg 1435 176 2 h
SC 10 mg/kg 760 32.17 8 h
IFN-γ16.8 IP 100 μg/kg 12.54 3.41 2 h (89)
SC 100 μg/kg 0.947 0.35 2 h
PEG-10- IFN-γ~26 IP 100 μg/kg 3.30 160.03 4 h (89)
SC 100 μg/kg 1.12 15.62 >24 h
PEG-20- IFN-γ~36 IP 100 μg/kg 6.4 179.5 4 h (89)
SC 100 μg/kg 2.3 29.52 >24 h
PEG-40- IFN-γ~56 IP 100 μg/kg 28.25 567.29 10 h (89)
SC 100 μg/kg 2.12 23.47 >24 h
Anti-CD20 mAb 145 IP 150 μg 106.63 0.195 ~24 h (90)
SC 150 μg 51.67 0.203 ~24 h
phFVIII 90–200 IP 50 U/kg NA ~300 2-4 h (91)
SC 50 U/kg NA # NA
ohVWF 500–2000 IP 50 U/kg NA ~250 2-4 h (91)
SC 50 U/kg NA # NA
GLP-1 3297
**
IP 50 nM 0.77 2.64 NA (84)
SC 50 nM 1.54 5.14 NA
*Units for AUC and C
max
are different among various macromolecules, however, they are the same within the same
study for different routes of administration (see details in the text where the specific study is discussed).
a
,%AUCin
comparison to AUC of IV administered MPEG-SOD. #, the protein was undetectable in plasma after SC administration.
NA, not available. **, MW in g/mol;
Pharm Res (2020) 37: 12 Page 9 of 17 12

To study the effect of dose and injection volume on phar-
macokinetics of IP administered macromolecules, Barrett and
colleagues injected two doses (2 and 100 μg) of IgG2
ak
(IgG
light chain, MW = 25 kDa) at two different injection volumes
(2 and 20 ml) in rats (86). Their observations revealed that
higher dose of administered IgG2
ak
results in higher
AUC and C
max
whereas, the higher volume resulted in
lower AUC and C
max
. According to the authors and
another research group (87), IP injection of higher vol-
ume induces diuresis and increased clearance of the
drug leading to a lower AUC.
In another study, comparative pharmacokinetics of AD-
114-PA600-6H (human single domain antibody against
CXCR4, MW = 60.6 kDa) at 10 mg/kg dose was studied in
mice after IV, IP and SC administration (88). The results
indicated higher AUC
last
after IP administration compared
to the SC, though the highest AUC was observed after IV
administration; AUC
last
for IV, IP and SC were 1871 μg.h/
mL, 1435 μg.h/mL and 760 μg.h/mL, respectively. The
C
max
were 467 μg/mL, 176 μg/mL and 32.17 μg/mL with
t
max
values of 1.8 min, 2 h and 8 h for IV, IP and SC admin-
istration of AD-114-PA600-6H, respectively.
A comprehensive pharmacokinetic study of IFN-γ(MW =
16.8 kDa, 100 μg/kg) and its PEG-ylated conjugates (with
PEG-10, −20 and −40 at 100 μg/kg dose) in rats showed
higher AUC
0-∞
and C
max
values after IP vs SC administration
for all IFN-γforms (89). In addition, the observed t
max
after IP
administration was 4 h, 4 h, 10 h and 2 h for PEG-10, PEG-
20, PEG-40-conjugated and intact IFN-γ, respectively, where-
as it exceeded 24 h for all PEG-ylated IFN-γand was 2 h for
native IFN-γafter SC administration. C
max
values were
160.03 ng/mL, 179.50 ng/mL, 567.29 ng/mL and
3.41 ng/mL after IP, and 15.62 ng/mL, 29.52 ng/mL,
23.47 ng/mL and 0.35 ng/mL after SC administration for
PEG-10, PEG-20, PEG-40-conjugated and intact IFN-γ,
respectively.
In another study, pharmacokinetics of anti-CD-20 mono-
clonal antibody veltuzumab (MW = 145 kDa, at 150 μg) was
studied in mice after IP and SC administration (90). The cal-
culated AUC was 106.639 nmole.h/mL after IP and 51.67
nmole.h/mL after SC administration. The observed C
max
were 0.195 and 0.203 nmole/mL for IP and SC administra-
tion, whereas t
max
was ~24 h for both routes.
In a recent study Shi and colleagues examined suitability of
IP and SC routes, in comparison to IV, for administration of
large plasma-derived proteins, coagulation factor VIII
(phFVIII, MW = 90–200 kDa, 50 U/kg) and von
Willebrand factor (phVWF, MW = 500–2000 kDa, 50 U/
kg) in mice (91). Both proteins were absorbed with t
max
of 2
to 4 h after IP administration, with observed C
max
of
~250 mU/mL for phVWF and ~300 mU/mL for phFVIII,
which is similar to plasma level of these proteins 2 to 4 h after
IV administration. On the contrary, phVWF and phFVIII
were undetectable after SC administration in mice, even
though another coagulation factor with a smaller molecular
size (FIX, MW = 55 kDa) was shown to be absorbed in signif-
icant amounts after SC administration in another study (92).
Notably, Shi and colleagues concluded that the hindered ab-
sorption of phVWF and phFVIII after SC was likely due to
their large molecular size retarding absorption through sub-
cutaneous capillaries and lymphatics.
It is noteworthy, that not all studies documented better
pharmacokinetic profile of IP vs SC administered ploypepti-
des or proteins. For example, in the study where Parkes and
colleagues observed higher AUC of exendin-4 after IP vs SC
administration, pharmacokinetics of glucagon-like peptide-1
(7–36) amide (also known as GLP-1) was also studied in the
rat (84). Their observations revealed lower AUC for GLP-1
after IP vs SC administration, documenting AUC of 0.77±
0.16 nM.h/ml and 1.54 ± 0.24 nM.h/ml, respectively.
Additionally, C
max
after IP and SC administration of 50 nmol
GLP-1 were 2.64 ± 2.11 nM and 5.14 ± 1.16 nM, respective-
ly. The authors attributed these results to potential biodegra-
dation of GLP-1 upon absorption from peritoneal cavity (84).
Collectively, based on the above-discussed experimental
studies, we can suggest that macromolecules of different mo-
lecular size get access to systemic circulation after IP adminis-
tration in intact/therapeutically active form and, in majority
of cases, systemic exposure of these molecules, i.e. AUC, is
higher after IP than SC route of administration (summarized
in Table IV).
BIOAVAILABILITY OF SUSPENSION
AND NANOPARTICLE FORMULATIONS
ADMINISTERED VIA IP ROUTE
In addition to solution formulations of small and macromole-
cules discussed above, experimental studies point out to bio-
availability of suspension formulations after IP administration.
Absorption of suspension preparations after IP administration
is mainly through lymphatic system and is primarily affected
by physicochemical properties of the pharmacological agent,
dissolution rate of the suspension in the peritoneal cavity and
particle size (93). For example, in a recent study Cardenas and
colleagues studied systemic bioavailability of 6-
methylcoumarin (water insoluble, MW = 160.17 g/mol) in
rats after IP and oral administration of 200 mg/kg suspension
prepared in Tween-80 and saline (94). The authors observed
more rapid absorption and higher bioavailability of the sus-
pension after IP vs oral administration, documenting t
max
val-
ues of 6 min and 30 min, and AUC of 2177.0 μg.min/ml and
977.2 μg.min/ml, respectively. On the other hand, systemic
bioavailability of 100 mg/kg elacridar suspension (water insol-
uble, MW = 563.6 g/mol), prepared in hydroxypropyl-
methylcellulose and Tween-80, after IP administration in
12 Page 10 of 17 Pharm Res (2020) 37: 12

mice was several fold lower compared to that of oral admin-
istration, with AUC of 90.30 μg.min/ml and 1460 μg.min/
ml, respectively (95). Notably, when the same drug was given
as a microemulsion (prepared with Cremophor EL, Carbiton
and Captex 355) at 10 mg/kg, systemic bioavailability of the
formulation after IP administration was several times higher
compared to oral administration, with AUC of 962 μg.min/
ml and 270 μg.min/ml, respectively (76). The authors con-
cluded that lower bioavailability of elacridar suspension com-
pared to emulsion was due to lower dissolution of the admin-
istered formulation in the peritoneal cavity, which contains
smaller amount of fluid compared to the gastrointestinal tract.
It is noteworthy, that dissolution of the suspension formulation
can be improved by vehicle selection. For instance, Sofia and
colleagues have compared the influence of different suspen-
sion vehicles on central effects of Δ9-tetrahydrocannabinol
(water insoluble, MW = 314.45 g/mol) at 10 and 40 mg/kg
doses after IP administration (96). Among four different
vehicles studied (bovine serum albumin-saline (BSA), 1%
Tween-80-saline, polyvinylpyrolidone-saline (PVP) and 10%
propylene glycol-1% Tween 80-saline (PG)), suspension of
Δ9-tetrahydrocannabinol in PG was the most effective fol-
lowed by PVP suspension. In a subsequent study, the same
investigators studied suspensions of Δ9-tetrahydrocannabinol
in the same four vehicles after IV, IP, oral, and SC adminis-
tration (97). The strongest CNS effects were observed with PG
suspension and the effects descended in the following order
IV > IP > SC > oral, based on the route of administration
(97). Lastly, particle size of the suspension formulation can also
affect the efficacy of IP administered drugs. The latter was
demonstrated by Ritschel and colleagues who studied toxicity
of pentobarbiturate suspension in 1% sodium-carboxymethyl
cellulose at two different particle sizes (<44 μm and 297–
420 μm) administered IP in mice, and observed about twice
higher toxicity of the drug at small vs large particle size sus-
pension (LD50 of 189 and 288 mg/kg, respectively) (98).
Another recent study investigated pharmacokinetics of two
poorly water soluble substances (at 5 μmol/kg, referred to as
AC88 and BA99) after IP administration in the rat, in the
form of microsuspension (prepared with hydroxylpropyl-
methylcellulose) and nanosuspension (prepared with Aerosol
OT, polyvinylpyrolidone and mannitol) (99). For both sub-
stances nanosuspensions showed about twice higher AUC
compared to that of microsuspensions. More specifically, the
observed C
max
,t
max
and AUC values were 8.24 μmol/L, 3 h
and 136 kg.h/L for AC88 microsuspension (mean size 14 μm),
and 21.2 μmol/L, 2 h and 233 kg.h/L for AC88 nanosuspen-
sion (mean size 219–251 nm). Similarly, for BA99 microsus-
pension (mean size 12 μm) the observed C
max
,t
max
and AUC
values were 10.4 μmol/L, 0.7 h and 50.6 kg.h/L, whereas for
BA99 nanosuspension (mean size 291 nm) the observed values
were 20.1 μmol/L, 0.31 h and 85.4 kg.h/L, respectively (99).
Notably, the same research group studied bioavailability of
AC88 and BA99 nanosuspensions after oral and SC adminis-
tration in the rat (at 5 μmol/kg) in two parallel studies, and
reported ~5–8 fold lower AUC values compared to IP admin-
istration of the same nanosuspensions (100,101).
It is important to note, that while the small particle size of
nanoformulations facilitates enhanced absorption of pharma-
cological agents, encapsulation of active substances in nano-
formulations often results in decreased clearance from sys-
temic circulation and ultimately higher drug exposure. In
addition, depending on specifics of nanocapsulated formula-
tion release of the drug may also be prolonged leading to
higher drug exposure. For example, pharmacokinetic com-
parison of lipid core nanocapsule formulation of olanzapine
(sparingly water soluble, MW = 312.4 g/mol) with free olan-
zapine after IP administration (at 10 mg/kg) indicated ~2.3
fold increased bioavailability of the nanoformulation in the
rat (102). In addition, higher C
max
and lower clearance (CL)
values were observed for olanzapine nanoformulation
(3.02 μg/ml and 1.36 L/h/kg) compared to the free drug
(1.12 μg/ml, and 3.12 L/h/kg). And because the observed
absorption rate was comparable for both formulations (t
max
~1 h) the authors concluded that the observed higher bio-
availability of nanoformulation was largely due to decreased
CL (103). In another study, chitosan-coated PLGA nanopar-
ticles of docetaxel (sparingly water soluble, MW = 807.87 g/
mol) were studied in the rat after IP administration (at
13 mg/kg) and ~4.7 fold higher bioavailability of the drug
was observed for the nanoparticle formulation in comparison
to free docetaxel suspension (103). Notably, the time to reach
maximum plasma concentrations (2 and 0.5 h) as well as the
elimination half-life (5.2 and 2.6 h) were higher for the nano-
formulation, indicating overall longer residence time for
docetaxel nanoparticle (7.4 h) in comparison to the free form
(4.3 h) (102). Similarly, in a recent study Ragelle and col-
leagues studied bioavailability of free (at 223 mg/kg) and
nanoemulsion (at 112.5 mg/kg, prepared with Miglyol®
812 N, Labrasol®, Tween-80, Lipoid® E80 and water) fise-
tin (sparingly water soluble, MW = 286.23 g/mol) after IP
administration in the rat (104). Their observations indicate
that bioavailability of fisetin nanoemulsion was about 24 fold
higher compared to the free drug, and it had to do with more
rapid absorption (1.97 and 5.98 h) and lower clearance (2.32
and 54.8 L/kg/h) of the nanoemulsion vs free drug, respec-
tively (104).
In summary, these experimental studies suggest that drugs
in suspension and/or nanoparticle formulations also reach
systemic circulation after IP administration. Notably, the
physicochemical properties of the drug, dissolution rate of
the suspension in the peritoneal cavity and particle size criti-
cally affect absorption rate and bioavailability of the adminis-
tered drug. In general, higher dissolution rate and smaller
particle size lead to more complete and rapid absorption of
such formulations from peritoneal cavity and result in higher
Pharm Res (2020) 37: 12 Page 11 of 17 12

bioavailability. From the other hand, slow dissolution rate
may lead to more prolonged absorption of the drugs from
such formulations and also result in longer exposure. Lastly,
encapsulation of active substances in nanoformulations may
prolong release of the pharmacological agent into systemic
circulation, and often leads to decreasedclearance of the drug.
LIMITATIONS OF THE IP ROUTE
Eventhough IP administration of pharmacological agents
results in faster and more complete absorption compared to
oral, intramuscular and SC routes, this route as any other, has
certain limitations. One limitation is the first pass metabolism,
similar to what is observed with orally administered drugs,
because substances absorbed from the peritoneal cavity end
up in portal vein and pass through the liver. It is generally
considered that, pharmacokinetics of small molecular drugs
administered through the IP route resemble that of orally
administered drugs in terms of metabolic fate and high rate
of first pass metabolism, which leads to lower systemic expo-
sure of IP-administered substances (29,75). Notably, published
literature does not allow to conclusively say whether the first
pass metabolism of IP-administered sustances is as expensive
as in case of the oral route and future studies will likely adress
this question. On the contrary, IP administered macromole-
cules, which reach systemic circulation through lymphatic ves-
sels, were shown to be minimally affected by first pass metab-
olism. This was well-documented with a number of recombi-
nant enzymes including neurolysin (80), angiotensin convert-
ing enzyme 2 (105) and SOD (83) which retained catalytic
activity upon reaching systemic circulation after IP adminis-
tration in rodents.
Two other important considerations for IP administered
agents include sterility and non-irritability, because irritating
compounds may cause ileus and peritoneal inflammation,
which may further develop into adhesions (106). Importantly,
position of IP injection can aslo affect the absorption rate of
substances. Although, not studied in experimental animals, a
study carried out in humans showed that the time to reach
maximum plasma concentration of insulin in healthy volunteers
varied about 2-fold upon administration of the agent at a posi-
tionabovevsbelowthetransversemesocolon(107). The tech-
nique of injection and its accuracy may also affect the outcome
of IP administration. One common mistake associated with IP
drug admnistration (~20% of cases) is puncturing the skin at a
very sharp angle which results in SC administration rather than
IP (108). Although occurring less frequently, inaccurate IP ad-
ministration may also deposit drugs into the gastrointestinal
tract, retroperitoneum or the urinary bladder (109). Another
important factor is the volume of IP administered drugs be-
cause large volumes (>10 ml/kg in rodents) can lead to pain,
chemical peritonitis, formation of fibrous tissue, perforation of
abdominal organs, hemorrhage, and respiratory distress
(86,110,111). Repeated IP administration can result in a cumu-
lative irritant effect and needle-induced damage of the perito-
neum (5). Temperature of the IP administered solutions can
also affect local absorption rate (112,113), whereas, hypother-
mia and distress can be observed if a large volume of a cold
substance is administered IP (114,115).
CONCLUDING REMARKS
Proper formulation of a drug and appropriate route of admin-
istration are crucial for clinical success at later stages of drug
development. However, such questions are usually addressed
after proof-of-concept studies where the goal is to evaluate the
effect(s) of target engagement rather than properties of a drug
formulation and/or its pharmacokinetics for clinical transla-
tion. Because of these reasons, in exploratory as well as early
preclinical studies it is more practical to choose a route of drug
administration that ensures bioavailability of the drug and
meets the needs of a specific experiment/scientific question
rather than focuses on clinically applicable administration
route. In this regard, IP route of administration can be the
choice for in vivo experimental studies in rodents, because it
is safe for animals, ensures therapeutic bioavailability of both
small and large molecules, amendable to variety of formula-
tions, suitable for chronic treatments, robust and easy to master.
Among all routes of drug administration the IV route usu-
ally results in the highest bioavailability of a drug. However,
this route is often impractical for rodent studies, because most
investigational pharmacological agents are difficult to fully
dissolve in water/aqueous solutions (hydrophobic nature), it
requires advanced skills to practice (especially in mice, due to
small vessels), and is not well-suited for chronic/repetitive
treatments. To avoid these problems, researchers often prefer
administration of pharmacological agents through oral, IP or
SC routes, all of which have certain advantages and disadvan-
tages (summarized in Table V). Among these, the IP route
stands out because the rate and extent of drug absorption is
faster in IP followed by intramascular > SC > oral routes
(116), and it is suitable for both drug solutions and suspen-
sions/emulsions. This is primarily because IP administered
pharmacological agents are exposed to a large surface area
(close to that of the entire skin surface) which leads to rapid
and efficient absorption. Notably, in majority of cases the
effect elicited by a pharmacological agent after IV administra-
tion can be approximated more closely with IP rather than
intramuscular or SC administration. Usually, the rate of ab-
sorption after IP administration is one-half to one-fourth as
rapid as after IV administration (66,69,117). Following rapid
absorption from the peritoneal cavity, a compound may face
one of the following two pathways to reach systemic circula-
tion: (1) it is absorbed through the visceral peritoneum, the
12 Page 12 of 17 Pharm Res (2020) 37: 12

mesentery and omentum and is drained into portal
circulation, or (2) the compound gets into the systemic circu-
lation directly bypassing liver when it is absorbed through
parietal peritoneum and lymphatics. Notably, small molecular
weight compounds are primarily absorbed through the first
pathway, because the surface area of membranes transporting
substances into the portal circulation is much larger (29,118).
On the contrary, macromolecules access systemic circulation
through the second pathway (lymphatics), which is very effi-
cient but relatively less recognized among researchers.
Another important point of consideration is the suitability
of IP route for chronic/repetitive treatments. For example,
studies have shown that chronic IP administration (daily saline
injection over 30 days in the same position of the abdomen)
and use of different types of injection vehicles and volumes are
safe and well-tolerated in laboratory animals (119,120).
From a technical standpoint, IP administrations is more
reproducible and easy to master compared to other available
routs, and upon mastering it is less stressful and safer for
rodents.
Among main deficiencies of the IP route, is the metabolic
fate of IP administered small molecular agents, which resem-
bles that of the orally administered drugs (121). This may
present a problem for pharmacological agents prone to exten-
sive first-pass metabolism.
In summary, based on the knowledge discussed in this
manuscript we conclude that IP administration of drugs in
experimental studies involving rodents is a justifiable route
for initial pharmacological and proof-of-concept studies
where the goal is to evaluate the effect(s) of target engagement
rather than properties of a drug formulation and/or its phar-
macokinetics for clinical translation.
Ta b l e V Brief Comparison of
Different Routes of Drug
Administration in Laboratory
Rodents
IP route
Advantages Disadvantages
Convenient and easy to master High first pass metabolism
Less stressful/harmful for animals Need for sterility
Suitable for variety of formulations (i.e., amendable to
physicochemical properties of a drug)
Abdominal organs can be injured if done incorrectly
Peritoneal cavity has buffering capacity Limited clinical applicability
Large volumes can be administered
Higher absorption rate
Suitable for repeated administrations
Suitable for administration of macromolecules
Oral Route
Advantages Disadvantages
Convenient and easy to master Can be stressful for animals
Suitable for repeated administrations Can damage the esophagus
Large volumes can be administered Low absorption rate and poor bioavailability
Clinically applicable Food and pH may affect absorption
High first pass metabolism
Gut flora may contribute to metabolism
Usually not suitable for biological agents
Intravenous Route
Advantages Disadvantages
No limitation in absorption Isotonic solution of a drug is required
Buffering capacity is present Need for sterility
Rapid onset of action Technically challenging
Suitable for administration of macromolecules Can be stressful for animals
Clinically applicable Not suitable for large volume administration
Challenging for repeated administrations
Subcutaneous Route
Advantages Disadvantages
Convenient and easy to master Low absorption rate and poor bioavailability
Less stressful/harmful for animals Not suitable for large volume administration
Suitable for most macromolecules Local irritability
Clinically applicable Need for sterility
Pharm Res (2020) 37: 12 Page 13 of 17 12

ACKNOWLEDGMENTS AND DISCLOSURES
Dr. Karamyan is supported by NIH (1R01NS106879) and
AHA (14BGIA20380826) grants. We apologize that the scope
of this manuscript prevented citation of all published experi-
mental studies where the IP route of drug administration was
used.
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