IN VIVO PK SCREENING - 1

v  Prior to clinical trials a new chemical entities’ (NCE’s) pre-clinical pharmacokinetics should be determined.  Once the test compound/NCE/drug molecule enters the body, it will undergo various processes including absorption via different administration routes, distribution into the body including different types of tissue, elimination via metabolic processes, as well as excretion via urine and feces.  It is possible to isolate and study some of these processes, by means of in vitro methods, and these methods will indeed be utilized prior to in vivo testing.  Due to the complexity of the entire body, in vivo tests cannot be completely avoided.

v  Before animal testing, the medicinal products are examined in a comprehensive manner using versatile in vitro methods, and only the most promising pharmaceutical compounds are selected for animal tests.  The basic assumption in all pre-clinical ADME studies is that the administered NCEs and dose levels are not toxic and do not cause any harm to the welfare of the animals.

Before talking about In Vivo ADME, let us re-cap In Vitro ADME and why is it important to perform In Vitro ADME before it’s In Vivo PK profiling.

In Vitro ADME

Prior to actual dosing in animals, a number of relatively rapid and cost effective in vitro assays can serve as surrogates and indicators of the ADME fate of compounds in vivo.

·         Evaluation of the pharmacological properties of Absorption, Distribution, Metabolism and Excretion of a test compound / candidate chemical leads are critical.

·         Once the ADME properties are evaluated then the next duty of a medicinal chemist is lead optimization while preserving the potency and selectivity of the chemical lead.

·         What one should keep in mind is sometimes more efficacious compounds have lower in vitro potencies, but better ADME properties.

Simply designing new analogs and developing a SAR for increased potency against the biological target is inadequate for the development of small molecule drugs.  Evaluation and optimization of structure-pharmacologic property-relationship (SPR) is a critical step for efficacy evaluation.

In addition to evaluating compound properties such as solubility, protein binding, serum stability, medicinal chemists team has to prioritize different structural classes and rank order them not only based on potency but also in relation to potential downstream absorption and metabolism liabilities.

Improvement in these ADME properties is sought prior to actual dosing in animals to assess PK, and certainly for larger compound efficacy studies, since animals are expensive and the ethics of sacrificing animals in poorly designed studies uninformed by pharmacological guidance are indefensible.

Usual physicochemical properties obtained from In Vitro ADME studies include,

Ø  Log D:  Provides lipophilicity information. 

Ø  Aqueous Solubility: Mostly Kinetic Solubility

Ø  Microsome Stability: Stability of the test compounds against liver microsomes, S9 fraction,   hepatocytes. Here we tend to get Human Liver Microsome (HLM), Mouse Liver Microsome (MLM), Rat Liver Microsome (RLM) stablity data

Ø  Plasma Stability:  Compounds stability in plasma.

Ø  Plasma Protein Binding:  How much of compound is bound to plasma protein?

Ø  Permeability:  Passive permeability information is obtained from PAMPA.  Other permeability      assays including Caco-2 and MDCK gives information related to active transport and P-gp efflux information.

Ø  Cytotoxicity: Most often hepatotoxicity information is what we look for.

Ø  CYP450 Inhibition:  Provides information related to possible drug-drug interaction.

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For in vivo PK studies, Association for Assessment and Accreditation of  Laboratory Animal Care (AAALAC)‐accredited animals, such as mice, rats,  dogs, and non‐human primates are employed to generate in vivo PK data like 

drug clearance (CL)

bioavailability (F%)

exposure (AUC)

half life (t1/2)

distribution volume (L)

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v It is highly recommended for PK/PD strategies to be implemented in early research phases of drug discovery projects to enable successful transition to drug development.

v Effective PK/PD study design, analysis, and interpretation can help scientists elucidate the relationship between PK and PD, understand the mechanism of drug action, and identify PK properties for further improvement and optimal compound design.

v Additionally, PK/PD modeling can help increase the translation of in vitro compound potency to the in vivo setting, reduce the number of in vivo animal studies, and improve translation of findings from preclinical species into the clinical setting.

After PK/PD studies are designed and conducted then the conclusions and interpretations are performed by

·        Pharmacology Laboratories – Pharmacodynamic portion of PK/PD studies, e.g., animal dosing and measurement of response

·        DMPK Laboratories – Measurement of concentrations and evaluation of pharmacokinetics

·        Other expert partners include, Formulation Department, Mathematical Modeling Department etcetera.