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A pharmacokinetic study provides the basis for determining drug exposures in the body over time. PK parameters are used in the evaluation of the absorption, distribution, metabolism, and excretion (ADME) processes of drugs.

In simpler terms, a pharmacokinetics study focuses on understanding the effect a body has on the administered drug product. By exploring individual ADME components, pharmacokinetics labs help clinicians prescribe the correct doses that will generate the highest benefit but with the least associated risk factors. Besides, pharmacokinetic studies aid in adjusting the drug doses depending on individual physiology and lifestyle. Hence, PK sample analysis is crucial for the success of a drug development project. From drug discovery and preclinical studies to PK clinical trials, pharmacokinetics assays are employed at every stage of drug development. Let us explore the importance of PK in clinical research through ADME studies.

Absorption

Absorption brings an administered drug product into the systemic circulation. Absorption affects the concentration and speed at which a drug reaches its target location. Clinical pharmacokinetic services employ different administration methods to generate clinical trial PK data. These routes of administration include oral, intramuscular, intravenous, subcutaneous, intrathecal, rectal, buccal, ocular, vaginal, inhaled, otic, transdermal, and nebulized. Each method has its advantages and limitations during PK analysis in pharmacokinetics clinical trials.

Absorption also includes liberating the active ingredient from the pharmaceutical drug form. Pharmacokinetics in clinical trials focuses on drug liberation, especially for oral medications. For example, the oral medication may reside in the throat or esophagus for an extended period after being ingested. This delay may damage the mucosa and impede the onset of drug effects. Besides, the low pH in the stomach may chemically react with the drugs even before they reach systemic circulation. Therefore, clinical pharmacokinetics services are concerned, particularly with drug absorption and liberation.

Distribution

Drug distribution varies from individual to individual. This variation is based on the drug properties and individual physiology. Hence, diffusion and convection are the primary factors affecting PK sample analysis in clinical trials. Moreover, these factors are affected by the size, binding kinetics, and polarity of the drug and the fluid status and body habitus of the patient. The primary aim of distribution in PK trials is to achieve effective drug concentration. For a drug product to be effective and active, it must reach its desired compartmental destination based on the volume of distribution and not protein-bound concentration.

Metabolism

Metabolism is the next step in a PK study clinical trial. It is the action of segregating the drug compound into components. Metabolism converts a drug compound into more water-soluble components and progresses them to renal clearance. It also metabolizes a prodrug into its active metabolites. Each body compartment, such as skin, plasma, gastrointestinal tract, lungs, and kidneys, has its metabolism strategy. PK labs and PK CROs focus on phase I and II reactions in the liver, as most of the metabolism occurs in the liver.

Excretion

Excretion is the final phase of PK evaluations of PK samples in clinical trials. It is through excretion that a drug is eliminated from the system. Generally, kidneys are common excretory organs. Although, certain drugs can be excreted through the skin, lungs, or gastrointestinal tract. However, pharmacokinetic CROs are always focused on complications associated with the reabsorption of specific compounds. In conclusion, Pharmacokinetics assays are essential in generating ADME data.

PK analysis in clinical trials is performed throughout the drug research and development process, starting from early discovery to the last Phase of drug development. The primary purpose of preclinical pharmacokinetic studies is to evaluate the characteristics of potential drugs to predict exposures and determine dose levels and frequencies for testing new chemical entities in preclinical disease efficacy models. PK studies in multiple species can be used to predict human pharmacokinetics and estimate the dose required for clinical efficacy and potential manufacturing costs for the intended drug product after achieving therapeutic proof-of-concept and honing structure-activity-relationships (SAR) to determine lead molecules. Pharmacokinetic (PK) Assays during the preclinical phase help determine bioavailability, the volume of distribution, half-life, and clearance. These PK studies help evaluate if the drug has adequate success potential or needs to be modified to improve its pharmacokinetic parameters. Pharmacokinetic (PK) study results from the preclinical stage help design IND enabling Tox studies in animals, and drugs can be advanced farther into clinical development based on these preclinical results. PK bioanalysis meeting the requirements for first in human (FIH) dosing initially undergo single ascending dose (SAD) clinical trials to assess the safety and clinical pharmacokinetics, followed by later multiple ascending dose (MAD) clinical trials to assess steady-state exposures and to correlate with drug pharmacology.

We perform PK analysis in clinical trials to examine the absorption, distribution, metabolism, and excretion of a drug. These studies are useful for determining the appropriate dose range for safety and efficacy in subsequent trials. Pharmacokinetic (PK) testing involves a quantitative analysis of the time course of a drug in the body. PK testing in clinical trials often assesses single ascending doses (SAD studies) to investigate dosage proportionality and multiple ascending doses (MAD studies) to correlate the PK effect changes with repeated dosage. SAD and MAD PK studies in clinical trials provide information regarding metabolic alterations and the achievement of steady-state exposures during chronic therapies. Other PK clinical trials include bioavailability and bioequivalence studies, assessment of drug-drug interaction (DDI) potential, and drug penetration studies depending on the therapeutic target.

Pharmacokinetic (PK) studies are crucial in understanding how drugs behave in the body over time. These studies evaluate several key components that directly relate to the ADME processes (Absorption, Distribution, Metabolism, and Excretion) of drugs. Here are the primary components evaluated in PK studies and their relation to ADME processes:

Key Components Evaluated in PK Studies

1. Absorption

• C_max (Maximum Concentration): The peak plasma concentration of a drug after administration, indicating the extent and rate of absorption.
• T_max (Time to Maximum Concentration): The time it takes for the drug to reach C_max, providing insight into the rate of absorption.
• AUC (Area Under the Curve): Represents the total drug exposure over time, indicating the extent of drug absorption.

2. Distribution

• Volume of Distribution (V_d): A theoretical volume that describes how extensively a drug is distributed throughout the body tissues relative to the plasma. It helps in understanding the extent of drug distribution.
• Plasma Protein Binding: The degree to which a drug binds to plasma proteins, affecting its distribution and free (active) drug concentration.

3. Metabolism

• Metabolic Clearance (Cl): The volume of plasma from which the drug is completely removed per unit time due to metabolic processes, primarily in the liver.
• Metabolite Profile: Identification and quantification of drug metabolites to understand the biotransformation pathways.

4. Excretion

• Renal Clearance: The volume of plasma cleared of the drug by the kidneys per unit time, indicating the rate of renal excretion.
• Half-Life (t_1/2): The time required for the plasma concentration of the drug to reduce by half, reflecting the combined effects of metabolism and excretion.
• Excretion Rate: The rate at which the drug and its metabolites are eliminated from the body, typically measured in urine or feces.

Relation to ADME Processes

• Absorption: Evaluated by C_max, T_max, and AUC. These parameters help in understanding how quickly and efficiently a drug enters the systemic circulation from the site of administration.
• Distribution: Assessed through volume of distribution and plasma protein binding. These factors determine how the drug is dispersed throughout body fluids and tissues, impacting its therapeutic and side effects.
• Metabolism: Studied via metabolic clearance and the metabolite profile. Metabolism transforms the drug into metabolites, which can be active or inactive, influencing the drug's efficacy and safety.
• Excretion: Evaluated through renal clearance, half-life, and excretion rate. These parameters describe how the drug and its metabolites are removed from the body, affecting the duration of the drug’s action and the frequency of dosing.

Understanding these components in PK studies allows researchers and clinicians to optimize drug dosing regimens, enhance therapeutic efficacy, minimize toxicity, and predict drug-drug interactions.

In early drug discovery, we often perform PK testing to determine whether new chemical entities (NCEs) have adequate exposures for in-vivo efficacy models in mice. Even before dosing in the animals, we typically perform in-vitro clinical pharmacokinetics services to filter out compounds with high metabolic rates or weak absorption potential. During preclinical development, pharmacokinetic testing in additional species is generally performed in one rodent and one non-rodent species to prepare for toxicology and toxicokinetics studies. Additional clinical pharmacokinetics services include allometry or other scaling models to predict clinical PK from the preclinical data ahead of the pharmacokinetics clinical trials. Early clinical PK studies are often performed at sub efficacious dosage with slow dose escalation to maintain safety and determine the clinical PK parameters while ramping towards therapeutic doses. For reformulations or generics of approved compounds, we perform PK testing to demonstrate bioequivalence. During all drug development phases, PK lab test results may be used to assess dose proportionality, steady-state exposure levels, bioavailability, and the general ADME properties of the test compounds in the various test species.

Pharmacokinetic (PK) study design follows some basic general principals even as it varies based on the chemical structures of the tested compounds and the intended therapeutic targets. Initially, we dissolve the test compound in an appropriate dosing vehicle. Then, the test species is dosed with the vehicle and blood samples are collected over a pre-specified period for drug concentration analysis. Afterward, we plot the blood draw time vs. concentration curves and calculate the PK parameters such as the area under the curve, maximum concentration, clearance, distribution volume, elimination half-life, etc. We select the species and strains for pharmacokinetic studies in animals to meet our objective towards proving compound efficacy and safety with the ultimate goal of reaching the clinic and marketplace. During drug discovery, PK studies in mice and rats are often performed in tandem with pharmacodynamic studies to achieve in-vivo proof of concept against a disease target. Late phase discovery and development PK studies are performed in multiple species to support preclinical toxicology and regulatory submissions.

Pharmacokinetic analysis provides direct information about the fate of a test compound in the body. Pharmacokinetics research focuses on the relationship between the dosing regimen and the body’s exposure to the drug. Typically, we generate PK data through non-compartmental analysis. Non-compartmental PK parameters include area under the curve (AUC), maximum exposure (Cmax), clearance (CL or CL/F), distribution volume (Vd or Vd/F), elimination half-life (t½), etc. These parameters provide crucial information about any drug’s absorption and elimination rates, as well as the potential to distribute to tissues throughout the body. Pharmacokinetics in drug development from multiple species can be used to predict dosing levels and regimens for efficacy in animal models of disease, for predictions of doses required for preclinical toxicology studies, and to scale predicted clinical PK parameters and estimate initial clinical doses.

Pharmacokinetics assays provide a quantitative description of what a body does to a drug. The four major components of the pharmacokinetics process are absorption, distribution, metabolism, and excretion commonly referred to as ADME. Absorption is the process of getting the drug from the site of delivery into the systemic circulation. For example, orally administered compounds must be soluble in the gut environment, then permeate the gastrointestinal tract and pass through the portal vein and the liver (known as the first pass) before reaching central circulation. Distribution describes the drug’s ability to enter and leave central circulation to reach tissues and organs throughout the body. Metabolism and excretion are the processes of eliminating drugs and xenobiotics from the body via metabolic processes or direct biliary or renal excretion of unchanged drug.

Pharmacodynamics describes what the drug does to the body, and pharmacokinetics describes what the body does to the drug. Pharmacokinetics (PK) and pharmacodynamics (PD) are both components of in-vivo pharmacology. Pharmacokinetics evaluates absorption, distribution, metabolism, and excretion of drugs by the determination of systemic exposure over time. Pharmacodynamics studies the relationship between drug concentration at the site of action (receptor) and the observed pharmacological response. PK describes drug exposure, and PD describes drug effects. PK PD analysis provides the relationship between the drug’s in-vivo exposure and efficacy. PK PD studies in multiple species during drug discovery and development are often useful for predictions of initial dosing levels and regimens for safety and efficacy in the clinic.

In-vivo PK drug assessments are critical in drug discovery, development, and clinical testing. Non-compartmental pharmacokinetic (PK) drug parameters vary depending on study design, but typically include volume of distribution (Vd) and total plasma clearance (CL) following IV doses, time to maximum concentration (Tmax), maximum observed concentration (Cmax), area under plasma concentration vs. time curve (AUC), elimination half-life (t1/2) for all dose routes, and bioavailability (F%) where extravascular and IV routes have been tested. As appropriate, our team of experienced scientists at NorthEast BioLab can aid you in study and protocol design, study monitoring, sample bioanalysis, pharmacokinetics parameter calculation using validated Phoenix WinNonlin software, and result interpretation during all project phases.