Toxicokinetics (TK) Study

Toxicology CROs offer predictive safety assessments by in-silico and in-vitro methods. In-silico methods allow predictions of potential mutagenicity and genotoxicity based on the compound’s structure and known historical data. In-vitro toxicity assays allow testing with many discovery compounds to predict specific toxicity mechanisms and reduce animal usage. In-vitro toxicity assays test for potential drug-drug interactions (DDI), mutagenicity, cytotoxicity, or organ toxicity using microsome, hepatocyte, hERG, and AMES assay. That said, in-vivo studies are still critical because they are the ultimate indicator of toxicity. In-vivo toxicity studies include satellite toxicity and maximum tolerated dose assessments, acute single-dose toxicity studies, multi-dose GLP toxicology studies, genetic and prenatal toxicology, carcinogenic studies, safety pharmacology studies, in-vivo erythrocyte micronucleus tests, and other systems of biology techniques including metabonomics, proteomics, and transcriptomics.

The toxicokinetics process encompasses the rate and extent of the four sequences that act on a compound in a biological system referred to as ADME. They include uptake of drug into the systemic circulation, or absorption (A), distribution (D) from the bloodstream to cells and tissues within the body, conversion to new entities by biotransformation or metabolism (M), and excretion (E) of the compound and its biotransformation products. Toxicity is often associated with the conversion of the drug compound to more reactive metabolites. We perform toxicokinetic analysis by assessing systemic exposures via bioanalysis followed by pharmacokinetics calculations using the in-vivo time vs. concentration results. Toxicokinetic studies allow determination of i) No observed effect level (NOEL) – the highest dose or exposure that produces no effect, ii) No observed adverse effect level (NOAEL) – the highest dose or exposure that produces manageable toxicity, and iii) Therapeutic index (margin of safety) – the range encompassing minimum efficacious dosage to maximum tolerated dose.

Preclinical GLP toxicology data must be obtained for all lead candidates before filing the Investigational new drug application (IND). In-vitro toxicology studies in drug development test mechanisms such as metabolic reactivity, inhibition, and induction, mutagenicity, and cytotoxicity, and results from these assays alert the scientists to potential problems or limitations. We perform in-vivo acute and chronic toxicology studies in drug development to optimize the lead discovery candidates. Furthermore, these days novel techniques including toxicometabonomics, toxicoproteomics, and toxicogenomics can provide higher throughput approaches to identify in-vivo toxic responses soon after dosing or aid in the selection of specific endogenous biomarkers for both clinical and preclinical detection of potential toxicities.

In-vivo toxicity studies are required before FIH (First In Human) dosing and are performed under GLP conditions using detailed protocols. A general scheme used in the industry for preclinical studies of lead compounds before the commencement of clinical trials is as follows: Acute (single-dose) toxicity, maximum tolerated dose (MTD), Chronic toxicity – 2 to 14 weeks (daily dosing, required by regulators in two species, one rodent and one non-rodent), carcinogenicity – up to 2 years of dosing in rodents for chronic treatments, genotoxicity- in vivo mouse micronucleus test, mutagenicity – in vitro AMES test, cardiovascular risk by hERG inhibition, chromosome aberration, and in-vivo respiratory and cardiovascular safety pharmacology studies, as applicable. The majority of these are GLP toxicology studies, following regulatory guidance with quality oversight, and often performed at a toxicology CRO.

Before conducting testing in animals, we must get approval for toxicity studies by the testing organization’s Institutional Animal Care and Use Committee (IACUC). Acute testing studies the effect of a single dose on an animal species. In acute toxicological testing, we administer the investigational product at different doses and observe the effects for ~14 days following a single dose. We perform non-GLP repeat dose toxicity studies in rodents (mice or rats) to identify maximum tolerated dose, target tissues, PK, and safety pharmacology profiles during late optimization and before the nomination of a compound for preclinical development. If the test agent has reached a steady-state, and there is a reasonable or no accumulation of test article and no observed acute toxicity at therapeutic levels, then we follow-up with repeat dose GLP nonclinical testing studies. In repeated dose toxicity studies, test drug is administered daily for a minimum of 7, 14, 21 & 28 days under GLP conditions. We perform subchronic toxicity studies in rodent and non-rodent by daily dosing for 90 days.

TK analysis is the subdivision of PK to test the relationship between systemic drug exposure and its toxicity in various species. Pharmacokinetic studies determine the systemic exposure of drug administration to living organisms over time after dosing. Toxicokinetic studies are generally carried out with higher doses as compared to pharmacokinetic studies. The objective of toxicokinetic studies is to quantify the pharmacokinetics of a compound in toxicological species at toxicological doses. TK exposures may increase non-linearly with dosage due to absorption differences, metabolic saturation, or metabolic alterations resulting from the chronic usage of the test article. TK analyses are often used additionally to investigate drug accumulation with repeated dosing, gender differences, and quantitation of known drug metabolites or pharmacodynamic or toxicodynamic biomarkers.