Intraperitoneal administration, commonly abbreviated IP or i.p., delivers drugs, fluids, or cells directly into the peritoneal cavity, the membrane-lined space surrounding the abdominal organs. The route exploits one of the body’s largest and most blood-rich internal surfaces to absorb substances into either the local tissue or the general circulation, depending on what a clinician or researcher needs. IP delivery is a mainstay in laboratory animal research, a critical tool in treating certain abdominal cancers, the basis of peritoneal dialysis for kidney failure, and an emerging option for insulin delivery in type 1 diabetes. Understanding why anyone would choose to put a drug into the belly instead of a vein or under the skin requires a closer look at what the peritoneum actually does once a substance lands on it.
The Peritoneum as a Drug-Absorbing Surface
The peritoneum is a thin, semi-permeable membrane that lines the abdominal wall and drapes over the intestines, liver, spleen, and other organs. Its total surface area in an adult is roughly comparable to the skin’s. Underneath the single-cell-thick lining sits a dense network of blood capillaries and lymphatic vessels, making the peritoneum surprisingly efficient at moving molecules from the abdominal cavity into the bloodstream.
Transport across the membrane happens through several pathways. Small molecules like salts and glucose cross mainly by diffusion through tiny gaps between the cells lining blood vessel walls. Water moves through those same gaps but also through a dedicated channel called aquaporin-1, a molecular pore that lets water through while blocking most dissolved substances. This “free water transport” is especially important during peritoneal dialysis, where a sugar-rich solution is used to pull excess water out of the body.1PubMed Central. Physiology of peritoneal dialysis; pathophysiology in long-term patients Larger molecules, including proteins the size of albumin, can also cross, though more slowly and through wider, less common pores.2PubMed. Solute transport across the peritoneal membrane
Modeling work suggests that in steady-state conditions, the majority of fluid entering the tissue from the peritoneal cavity is taken up by blood capillaries rather than lymphatic vessels, with capillaries accounting for roughly 60 to 80 percent of absorption and local lymphatics handling the rest.3American Journal of Physiology-Heart and Circulatory Physiology. Distributed model of peritoneal fluid absorption That split matters for drug design: substances meant to reach the bloodstream quickly benefit from capillary uptake, while those intended to stay in the abdomen or travel through the lymphatic system need to be engineered differently.
Where the Drug Goes After an IP Injection
One of the defining features of IP delivery is the first-pass effect through the liver. A large share of what is absorbed through the visceral peritoneum, the layer covering the intestines and other organs, enters the portal vein and flows directly to the liver before reaching the rest of the body.4Surgical Oncology Clinics of North America. Impact of perioperative intraperitoneal chemotherapy on the treatment of primary gastric cancer For some drugs, this is a disadvantage: the liver breaks them down before they ever reach their target elsewhere. For others, it is a deliberate strategy. A chemotherapy agent that is metabolized into a nontoxic form during its pass through the liver can achieve very high concentrations against tumors lining the peritoneal cavity while producing comparatively low levels of drug in the general circulation, reducing side effects.
Animal studies illustrate this pharmacokinetic advantage clearly. When the chemotherapy drug irinotecan was given to mice by IP injection instead of intravenously, the drug concentration in the peritoneal fluid was substantially higher with the IP route, while plasma levels stayed about the same. Peritoneal clearance of the drug was roughly ten times slower after IP injection compared with IV, meaning the drug lingered far longer where it was needed.5PubMed. Comparison of the pharmacokinetics and efficacy of irinotecan after administration by the intravenous versus intraperitoneal route in mice
Particle Size Dictates the Journey
Not everything injected into the peritoneal cavity follows the same absorption path. Particle size plays a decisive role. Research on polymeric nanocarriers ranging from 30 to 1,000 nanometers in diameter has shown that small particles, around 30 nanometers or below, are rapidly absorbed and achieve high concentrations in the bloodstream, making them useful for systemic therapy. Larger particles, over 1,000 nanometers, tend to stay in the abdominal cavity, making them better suited for treating local diseases like peritoneal cancers. Mid-range particles follow lymphatic channels and accumulate in lymph nodes, a property being explored for targeting cancers that have spread through the lymphatic system.6PubMed. Intraperitoneal drug delivery systems for peritoneal carcinomatosis: Bridging the gap between research and clinical implementation This size-dependent behavior offers a kind of built-in routing system: by choosing particle size, researchers can steer a drug toward the bloodstream, the lymph nodes, or the peritoneal cavity itself.7PubMed Central. Hydrogel-Based intraperitoneal drug delivery platforms for peritoneal metastasis: strategies, advances, and prospects
What You Mix the Drug In Matters
The carrier solution used to dissolve or suspend a drug for IP delivery has a meaningful effect on how long the drug stays in the abdomen. Hypotonic and isotonic saline solutions tend to be absorbed from the peritoneal cavity relatively quickly, carrying the drug with them. Hypertonic solutions and high-molecular-weight carriers like hetastarch and icodextrin keep fluid in the cavity much longer, because they draw water across the membrane in the opposite direction or resist absorption themselves. In one study comparing several carriers, the volume remaining in the peritoneal cavity at six hours was significantly greater with hetastarch and icodextrin than with plain isotonic saline for both of the chemotherapy drugs tested.8PubMed. Impact of carrier solutions on pharmacokinetics of intraperitoneal chemotherapy
This is clinically relevant for IP chemotherapy, where the goal is prolonged drug contact with tumor-bearing peritoneal surfaces. Using a carrier that keeps the solution in place longer translates to more drug exposure at the tumor site. Interestingly, while solution osmolality affects fluid dynamics, its effect on how deeply a drug penetrates into solid tumors sitting on the peritoneal wall is limited. Experiments in rats found that switching from isotonic to hypertonic IP solutions changed antibody concentrations only near the tumor surface, not deeper within the tissue.9PubMed. Monoclonal antibody delivery to intraperitoneal tumors in rats: effects of route of administration and intraperitoneal solution osmolality So carrier selection can extend dwell time, but it does not automatically improve drug penetration into bulky tumors.
IP Chemotherapy for Abdominal Cancers
The most prominent clinical use of IP drug delivery is in treating cancers that spread along the peritoneal lining, particularly ovarian cancer and cancers originating from the colon, stomach, or appendix. The logic is straightforward: if the cancer is coating the inside of the abdomen, bathing those surfaces in chemotherapy should deliver higher drug concentrations to the tumors than an IV infusion that must first circulate through the entire body.
Hyperthermic intraperitoneal chemotherapy, or HIPEC, pairs this approach with heat. During surgery to remove visible tumor from the abdominal cavity, the surgeon perfuses a heated chemotherapy solution directly over the peritoneal surfaces. In a landmark trial involving patients with advanced ovarian cancer, adding HIPEC to cytoreductive surgery extended median recurrence-free survival from about 10.7 months to 14.2 months and median overall survival from roughly 34 months to nearly 46 months. The risk of disease recurrence or death dropped by about a third compared to surgery alone.10PubMed Central. Hydrogel-Based intraperitoneal drug delivery platforms for peritoneal metastasis: strategies, advances, and prospects Those are meaningful gains for a cancer that is notoriously difficult to control once it has spread across the peritoneum.
Peritoneal Dialysis
While oncologists use the peritoneum to deliver drugs, nephrologists use it as a filter. Peritoneal dialysis is a home-based alternative to hemodialysis for people with kidney failure. A permanent catheter placed in the abdomen allows the patient to fill the peritoneal cavity with a specially formulated solution, typically containing a high concentration of glucose. The glucose creates an osmotic gradient that pulls water and waste products out of the blood, across the peritoneal membrane, and into the solution, which is later drained and replaced.
The capillary wall is the main bottleneck for transport during peritoneal dialysis. Small waste molecules like urea cross primarily by diffusion through gaps between endothelial cells, while water removal depends heavily on the osmotic gradient driving flow through aquaporin-1 channels.11American Journal of Physiology-Renal Physiology. Peritoneal dialysis: from bench to bedside and bedside to bench A complicating factor is that the peritoneal membrane absorbs fluid at the same time ultrafiltration is pulling fluid out, so the net water removal is always less than you might expect from the osmotic gradient alone.12PubMed. Solute transport across the peritoneal membrane
Over years of use, the peritoneal membrane gradually changes. Chronic exposure to glucose-based dialysis solutions can cause the membrane to become thicker and more fibrotic, with new blood vessels growing into it. These changes tend to increase small-solute transport (which sounds helpful but actually means dialysis fluid loses its osmotic pull faster) and decrease the membrane’s ability to remove water effectively. In rare but serious cases, long-term dialysis can lead to encapsulating peritoneal sclerosis, a condition where the bowel becomes wrapped in a thick cocoon of scar tissue.13Peritoneal Dialysis International: Journal of the International Society for Peritoneal Dialysis. Experimental Animal Models of Encapsulating Peritoneal Sclerosis
IP Insulin Delivery for Type 1 Diabetes
A less well-known but genuinely promising application of IP delivery is insulin infusion for people with type 1 diabetes. When insulin is injected under the skin, the standard approach, it enters the general circulation and reaches the liver only after passing through the entire body. The result is that peripheral tissues see unnaturally high insulin levels while the liver, which is supposed to be insulin’s primary target, gets a diluted signal. IP insulin delivery reverses this problem. Because the peritoneum drains largely into the portal vein, IP insulin reaches the liver first, mimicking the normal physiology of a working pancreas.14Journal of Diabetes Science and Technology. Intraperitoneal Insulin Delivery: Evidence of a Physiological Route for Artificial Pancreas From Compartmental Modeling
The clinical results bear this out. A systematic review and meta-analysis comparing continuous IP insulin infusion with subcutaneous pump delivery found that IP insulin lowered HbA1c (a measure of long-term blood sugar control) by about 0.6 percentage points more than subcutaneous delivery. Severe episodes of both dangerously high and dangerously low blood sugar were less frequent, and fasting insulin levels in the blood were lower, reflecting reduced peripheral hyperinsulinemia.15PLOS ONE. Physiological effects of intraperitoneal versus subcutaneous insulin infusion in patients with diabetes mellitus type 1: A systematic review and meta-analysis In a pilot study using an automated artificial pancreas system, IP delivery kept blood sugar in the ideal range of 80 to 140 mg/dL roughly 40 percent of the time, compared with about 26 percent for subcutaneous delivery. Time spent in hyperglycemia above 250 mg/dL dropped from about 23 percent to roughly 6 percent, a dramatic reduction, without any increase in dangerous lows.16PubMed Central. Intraperitoneal insulin delivery provides superior glycaemic regulation to subcutaneous insulin delivery in model predictive control-based fully-automated artificial pancreas in patients with type 1 diabetes: a pilot study
The trade-off is that IP insulin requires an implanted pump with a catheter in the peritoneal cavity, which introduces surgical risks, the possibility of catheter blockage, and the same infection and fibrosis concerns that affect peritoneal dialysis catheters. For now, IP insulin remains available primarily in specialized centers, but the physiological advantages are strong enough that research into making the hardware more practical continues.
CAR T Cells and Next-Generation IP Therapies
Some of the most exciting recent work in IP delivery involves putting living cells, not just drugs, into the peritoneal cavity. CAR T-cell therapy, in which a patient’s own immune cells are engineered to recognize and attack cancer, has shown remarkable results in blood cancers. Solid tumors have been far harder to treat this way, partly because T cells infused intravenously struggle to concentrate at the tumor site. Injecting CAR T cells directly into the peritoneum, where the tumor is growing, offers a workaround.
Preclinical studies have been striking. In animal models of peritoneal cancer from gastroesophageal and colorectal origins, IP delivery of CAR T cells at low doses produced stronger tumor killing, longer survival, and more persistent T-cell activity compared to IV delivery.17PubMed Central. Intraperitoneal CAR T-cell therapy for peritoneal carcinomatosis from gastroesophageal cancer: preclinical investigations to a phase I clinical trial IP-delivered CAR T cells were not only effective against tumors in the abdomen but also showed activity against tumors at distant sites outside the peritoneal cavity, suggesting that the cells can migrate from their injection site to hunt down cancer elsewhere.18PubMed. Intraperitoneal administration of carcinoembryonic antigen-directed chimeric antigen receptor T cells is a robust delivery route for effective treatment of peritoneal carcinomatosis from colorectal cancer in pre-clinical study In an ovarian cancer model, a single IP injection of CAR T cells targeting a surface marker called EpCAM was enough to eliminate established tumors and significantly extend survival.19PubMed Central. Intraperitoneal immunotherapy with T cells stably and transiently expressing anti-EpCAM CAR in xenograft models of peritoneal carcinomatosis
Hydrogel-based delivery platforms are another frontier. Instead of injecting a drug in solution, which washes away or gets absorbed relatively quickly, a hydrogel can act as a depot that slowly releases drug over days or weeks, maintaining high local concentrations with less frequent dosing. These systems are being developed for chemotherapy, immunotherapy, and combination approaches, and they may help solve one of IP therapy’s persistent challenges: keeping the drug where it needs to be for long enough to do its job.20PubMed Central. Hydrogel-Based intraperitoneal drug delivery platforms for peritoneal metastasis: strategies, advances, and prospects
Practical Considerations in Laboratory Animals
IP injection is one of the most common routes of administration in preclinical research, particularly in mice and rats. It allows rapid delivery of relatively large fluid volumes compared to other routes, and it does not require the technical skill needed to hit a tail vein. But the technique has its own pitfalls.
Where exactly the needle goes matters. Anatomical studies have shown that the cecum, a large pouch of intestine, typically sits on the left side in rats and mice. To avoid accidentally puncturing it, the recommended injection site is the right lower quadrant of the abdomen.21Laboratory Animals. Caecum location in laboratory rats and mice: an anatomical and radiological study Even with proper technique, misinjections happen. A study evaluating injection accuracy in mice found that the stress response from IP injection, measured by heart rate increase, could be more pronounced than from subcutaneous or intramuscular injections, though this difference was inconsistent depending on the injection fluid used.22Laboratory Animals. Effect of restraint and injection methods on heart rate and body temperature in mice Researchers designing experiments need to account for both the physiological stress of the injection itself and the possibility that some fraction of doses will end up outside the peritoneal cavity, potentially in the bowel wall, retroperitoneal fat, or even subcutaneous tissue.
Infection and Catheter Complications
Any time a catheter sits in the peritoneal cavity for weeks or months, as in peritoneal dialysis or implanted insulin pumps, infection becomes a constant concern. Peritonitis, infection of the peritoneal cavity, remains the most common serious complication of peritoneal dialysis. Bacteria frequently colonize the catheter itself, forming biofilms that are highly resistant to antibiotics. In one study of catheters removed because of infection, nearly 90 percent showed bacterial biofilm on the surface.23PubMed Central. Bacterial biofilm formation of peritoneal dialysis catheter in patients with peritonitis-associated catheter removal Once a biofilm establishes itself, it acts as a persistent reservoir that can seed repeated infections, sometimes necessitating catheter removal.24PubMed. Update on the challenging role of biofilms in peritoneal dialysis
Beyond infection, chronic IP administration of irritating substances can provoke peritoneal fibrosis, a thickening and scarring of the membrane that progressively degrades its transport function. Experimental animal models of this process use repeated IP injections of chemical irritants to reproduce the progressive inflammation, collagen deposition, and eventually the encapsulating “cocoon” formation seen in the most severe human cases.25Peritoneal Dialysis International: Journal of the International Society for Peritoneal Dialysis. Experimental Animal Models of Encapsulating Peritoneal Sclerosis Strategies to preserve peritoneal health over long-term use remain an active area of research, including work on more biocompatible dialysis solutions and anti-fibrotic agents.
Veterinary Applications
IP fluid administration is not limited to humans and lab rodents. In veterinary medicine, it offers a practical alternative when intravenous access is difficult or impractical. Neonatal calves, for instance, are often born dehydrated, and placing an IV line in a tiny, uncooperative animal on a farm can be challenging. Research has confirmed that saline administered intraperitoneally to newborn calves is effectively absorbed into the bloodstream within a clinically useful timeframe, making IP fluid therapy a viable option for moderately dehydrated animals.26PubMed. Intraperitoneal fluid administration to neonatal calves
The same rationale applies in high-volume shelter settings, where performing IV catheterization on every cat undergoing spay surgery is not always feasible. A recent study evaluated IP fluid administration during ovariohysterectomy in pregnant cats and found it to be a safe approach for maintaining hydration when IV access was not readily available.27Journal of the American Veterinary Medical Association. Safe use of intraoperative intraperitoneal crystalloid fluid administration in management of late-term pregnant queens undergoing elective ovariohysterectomy These veterinary use cases underscore a broader point about IP delivery: its value scales with difficulty of access. When veins are small, fragile, or practically unreachable, the peritoneal cavity provides a surprisingly effective plan B, one that has been used across species for decades but continues to see its evidence base refined and expanded.