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

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.

Practical approaches to evaluating and optimizing brain exposure in early drug discovery

Developing drugs for CNS related diseases continues to be one of the most challenging endeavors in drug discovery. This is at least in part related to the existence of the Blood Brain Barrier (BBB), a complex multicellular organization that provides selective access to required nutrients and hormones, while removing waste and restricting exposure to potential harmful toxins, pathogens, and xenobiotics. Consequently, designing and selecting molecules that can overcame this protection system are unique and critical aspects of the CNS drug discovery. Here we review modern CNS pharmacokinetic concepts and methods suitable for early drug discovery, and medicinal chemistry strategies towards molecules with optimal CNS exposure.

Use of Subcutaneous and Intraperitoneal Administration Methods to Facilitate Cassette Dosing in Microdialysis Studies in Rats

Microdialysis is a powerful technique allowing for real-time measurement of unbound drug concentrations in brain interstitial fluid in conscious animals. Use of microdialysis in drug discovery is limited by high resource requirement and low throughput, but this may be improved by cassette dosing. Administering multiple compounds intravenously of diverse physiochemical properties, it is often very challenging and time consuming to identify a vehicle that can dissolve all of the compounds. To overcome this limitation, the present study explores the possibility of administering a cassette dose of nine diverse compounds (carbamazepine, citalopram, desmethylclozapine, diphenhydramine, gabapentin, metoclopramide, naltrexone, quinidine, and risperidone) in suspension, rather than in solution, by intraperitoneal and subcutaneous routes, and determining if this is a viable option for assessing blood-brain barrier penetration in microdialysis studies. Repeated hourly subcutaneous dosing during the 6-hour microdialysis study allowed for the best attainment of distributional equilibrium between brain and plasma, resulting in less than a 2-fold difference in the unbound brain to unbound plasma concentration ratio for the cassette dosing method versus discrete dosing. Both subcutaneous and intraperitoneal repeated dosing can provide a more practical substitute for intravenous dosing in determining brain penetration of a cassette of diverse compounds in brain microdialysis studies. The results from the present study demonstrate that dosing compounds in suspension represents a practical approach to eliminating the technical challenge and labor-intensive step of preparation of solutions of a mixture of compounds and will enable the use of the cassette brain microdialysis method in a central nervous system drug discovery setting.

Intracerebral microdialysis in blood-brain barrier drug research with focus on nanodelivery

Microdialysis has contributed significantly to advance the understanding of BBB transport of drugs and to reveal key aspects of BBB transport, including quantifying active efflux and active uptake. Microdialysis studies on pharmacokinetic-pharmacodynamic relationships have given in-depth understanding of the processes involved. Recently, nanodelivery to the brain has been investigated with microdialysis, contributing to nanodelivery science by giving quantitative information on the possible success of different delivery vehicles and how they are involved in BBB transport.

An ex Vivo Model for Evaluating Blood-Brain Barrier Permeability, Efflux, and Drug Metabolism

The metabolism of drugs in the brain is difficult to study in most species because of enzymatic instability in vitro and interference from peripheral metabolism in vivo. A locust ex vivo model that combines brain barrier penetration, efflux, metabolism, and analysis of the unbound fraction in intact brains was evaluated using known drugs. Clozapine was analyzed, and its major metabolites, clozapine N-oxide (CNO) and N-desmethylclozapine (NDMC), were identified and quantified. The back-transformation of CNO into clozapine observed in humans was also observed in locusts. In addition, risperidone, citalopram, fluoxetine, and haloperidol were studied, and one preselected metabolite for each drug was analyzed, identified, and quantified. Metabolite identification studies of clozapine and midazolam showed that the locust brain was highly metabolically active, and 18 and 14 metabolites, respectively, were identified. The unbound drug fraction of clozapine, NDMC, carbamazepine, and risperidone was analyzed. In addition, coadministration of drugs with verapamil or fluvoxamine was performed to evaluate drug-drug interactions in all setups. All findings correlated well with the data in the literature for mammals except for the stated fact that CNO is a highly blood-brain barrier permeant compound. Overall, the experiments indicated that invertebrates might be useful for screening of blood-brain barrier permeation, efflux, metabolism, and analysis of the unbound fraction of drugs in the brain in early drug discovery.