Introduction to Pharmacokinetics

Transcription

1 Introduction to Pharmacokinetics J. Scott Daniels, Ph.D. Director, DMPK Vanderbilt Center for Neuroscience Drug Discovery Assistant Professor, Pharmacology Vanderbilt Univ Medical Center

2 History s first human drug metabolism experiment early 1800s Friedrich Woehler Alexander Ure Chemical transformations in vivo (biotransformation) Gout treatment W. Keller Human subject

3 What is pharmacokinetics (PK)? Why do we need PK? How do we do PK? How is the data generated? How do we do PK analysis?

4 PHARMACMETRICS Quantitative description of pharmacology Science Models Pharmacology Pharmacokinetics Pharmacodynamics Experiment Dose Concentration Effect

5 DEFINITINS Pharmacokinetics describes the movement (Greek kinesis) of a drug (Greek pharmakon) around the body Pharmacokinetics is the study of the rates of absorption, distribution, metabolism and excretion of a drug and its metabolite(s). Methods such as Statistical Moments, Sums of exponential modeling and Physiologically based kinetic modeling are used. Toxicokinetics is pharmacokinetics studied at high doses (toxic?) Clinical Pharmacokinetics is the application of pharmacokinetic principles to the therapeutic management of patients Population Pharmacokinetics uses advanced statistical methods and fragmented and sparsely available data to determine PK parameters and their associated variability

6 What is pharmacokinetics (PK)? Why do we need PK? How do we do PK? How is the data generated? How do we do PK analysis?

7 DRUG DISPSITIN Pharmacokinetics tries to answer the questions: Why does only a fraction of the total dose reach its target? How should we dose (route) and how many times (frequency) to maintain drug at target (efficacy)?

8 PRE-CLINICAL UTCMES FRM DING PK Select compounds that have the maximum potential of reaching the target (PK) Select the appropriate route of administration to deliver the drug Understand how the blood (or plasma) levels relate to efficacy (PK-PD) or toxicity (TK-TD) in order to select safe doses Decide on the frequency and duration of dosing in order to sustain drug at target for disease modification Predict Human pharmacokinetics

9 What is pharmacokinetics (PK)? Why do we need PK? How do we do PK? How is the data generated? How do we do PK analysis?

10 PERFRMING A PK STUDY DSE CLLECT SAMPLES (BLD, URINE, BILE, FECES) AT VARIUS TIMEPINTS ANALYZE FR DRUG/METABLITES PK DATA ANALYSIS

11 Rodent PK Screening Design Cassette dosing can enable rapid SAR on clearance (CLp)! Confirm cassette PK parameters with discrete dosing design

12 What is pharmacokinetics (PK)? Why do we need PK? How do we do PK? How is the data generated? How do we do PK analysis?

13 Plasma Concentration vs Time Curves mass spectrometer HTS autosampler HPLC Electrospray ionization MS revolutionized drug discovery! (plasma, urine, bile analysis)

14 What happens to a drug when taken orally (or iv, ip, sc)? Pharmacokinetics (PK) the mathematics of the time course of Absorption, Distribution, Metabolism, and Excretion of drugs in the body. A favorable PK profile is vital to the therapeutic success of a drug Drug must be able to reach its intended target

15 Plasma Concentration vs Time Curves addresses efficacy addresses safety

16 CNC (mass/volume) CNC (mass/volume) FEATURES F AN IV BLUS PK CURVE C = C 0 * e -kt C = Ae - t + Be - t 1 C max Concentration at time = 0 C max not observed only calculated Terminal Phase Slope of the line = ke or = Elimination rate constant t1/2 = 0.693/ke( ) As time infinity Conc Pre-terminal Phase Distribution and Elimination TIME Mono-exponential Bi-exponential TIME Mono-exponential Bi-exponential Drug experiences only distribution and elimination C max is not observed, it is calculated (by extrapolation) as the Conc at t=0 The terminal phase half-life is the true elimination half life of the drug

17 CNC FEATURES F AN RAL PK CURVE 3.5 Cmax Ka >> Ke Absorption is not rate limited Terminal phases are parallel Same Dose = Same AUC Tmax TIME Ka = 2 Ka = 1 Ka = 0.5 Ka = 0.2 Usually Absorption is faster than elimination i.e. Ka >>Ke As Ka approaches Ke; C max and T max (for same Dose)

18 Intravenous Advantages: No absorption stage - immediate effect without delay Guaranteed 100% bioavailability - no variation between patients Disadvantages: Person administering dose needs careful training Sterility essential Possible extravasation

19 Extravascular administration Any route other than i.v. An absorption stage will be involved Bioavailability may not be 100% Principally oral but also intramuscular, subcutaneous etc

20 Mouth Blood drainage from G.I.T. General circulation Stomach Small intestine Large intestine Liver Rectum General circulation

21 Extravascular administration Almost pure elimination

22 ral Advantages: Simplicity Disadvantages: Low and unpredictable bioavailability for some drugs Rate of absorption - slow and unpredictable Release from the tablet/capsule etc Gastric emptying

23 Intramuscular il or water Drug Blood Rate of absorption of drug may be limited either by rate of release from injection vehicle or the ability of the blood to carry the drug away.

24 Subcutaneous Drug has to carried away from the injection site by blood or lymph. Both blood and lymph flows to the subcutaneous tissue are poor. Release therefore rather slow. If slow release is wanted, probably better to use a slow release formulation (il or plastic). Can be painful.

25 Topical With topical application, intention is generally to achieve a local effect. However, in many cases, drug will be absorbed into the general circulation. Examples: Inhaled steroids intended to act in the lungs, but are absorbed and can cause some adrenal suppression. Beta-blockers in eye-drops reach measurable concentrations in blood.

26 IV vs. RAL IV Drug can be accurately dosed with a high level of control No absorption and hence bioavailability, first pass etc. is not an issue Rapid availability of drug for efficacy. No delays Not practical Patient compliance = 0 Requires trained medical personnel for administration Extravasation can cause severe local toxicity nce administered there is no recall RAL Convenient and Safe Large surface area for absorption Food and varying ph at different parts of the GI tract can facilitate absorption Less abrupt change of drug concentrations than with parenteral administration First pass metabolism by the liver Relatively slow onset of action Absorption can be rate-limited by large particle size and poor dissolution Sensitivity to acid or digestive enzymes Presence, type and temperature of food Gastric emptying time Intestinal motility

27 What is pharmacokinetics (PK)? Why do we need PK? How do we do PK? What do the data look like? How do we do PK analysis?

28 PK DATA ANALYSIS NN CMPARTMENTAL METHDS (STATISTICAL MMENTS) CMPARTMENTAL ANALYSIS (SUM F EXPNENTIALS MDELING)

29 CNC (mass/volume) CMPARTMENTAL ANALYSIS Determine shape of profile on semi-log scale Evaluate the # of kinetic phases observed CMPT. BDY MDEL Select a 1 or 2 or 3 exponential equation based on # of kinetic phases CMPT. BDY MDEL Best fit the observed data to the model using statistical goodness of fit criterion TIME Mono-exponential Bi-exponential WinNonLin 5.3 BTAIN PK PARAMETERS MDEL

30 CNC (mass/volume) CNC (mass/volume) CMPARTMENTAL MDELING (TW CMPARTMENTS) Dose k CENTRAL 12 PERIPHERAL Vc 1 V T 2 k 21 k 10 TW CMPARTMENT MDEL TIME Rapid distribution to highly perfused tissues characterized by alpha phase with a distributive half-life t 1/2 ( ) 1 TW CMPARTMENT MDEL alpha After distribution equilibrium, terminal beta phase representing elimination with an elimination half-life t 1/2 ( ) C A e α t Be β t beta TIME

31 CNC (mass/volume) NN-CMPARTMENTAL ANALYSIS (rapid approach) Determine shape of profile on semi-log scale Perform linear regression on terminal linear (hopefully) phase DSE V k e Determine slope of the line. This the elimination rate constant Ke 0.01 SLPE F TERMINAL PHASE Estimate AUC (Area Under the Curve) and AUMC (Area Under the first Moment Curve) by trapezoidal rule WinNonLin DERIVE TIME Volume of distribution, Clearance, half-life (terminal phase only), Mean Residence Time

32 CNCN. (mass/volume) AREA UNDER THE CURVE TRAPEZIDAL RULE 9 8 AUC last = Σ AUC AUC = C n AUC 0 C 2 last n 1 C*dt * t n C K n t last e n 1 C i 1 1 i TRAPEZID Measure of systemic exposure AUC proportional to Dose = Linear pharmacokinetics TIME Cl, Vd, MRT, t 1/2

33 VLUME(S) F DISTRIBUTIN (V) V app = Amount of drug in body at equilibrium Drug Conc. in plasma (blood) No physiological significance but its magnitude gives a general idea of extent of distribution in the body Monoexponential kinetics are explained by a single volume (Vd) Multi-exponential kinetics yield an instantaneous volume (Vc), a terminal volume (Vβ) and a steady state volume (Vss) Blood Tissue

34 VLUMES F DISTRIBUTIN: Examples Vd, (human, L/kg) Warfarin 0.1 Gentamicin 0.23 Theophylline 0.5 Cimetidine 2 Digoxin 7.3 Mianserin 13 Quinacrine 714 Small vol. Mainly stays in plasma little in tissues. Medium vol. Similar concs in plasma and tissues Large vol. Mainly in tissues, little in plasma.

35 CLEARANCE (Cl) Defined as the volume of blood that is completely cleared of the drug/unit time when it passes through a clearing organ Define clearance mech and improve stability (CL int, in vitro) Cl Dose AUC HEPATIC EXTRA-HEPATIC METABLIC BILIARY RENAL PULMNARY For linear pharmacokinetics:- Cl is constant as a function of dose THER Cl is additive i.e. Cl tot = Cl hepatic + Cl extra-hepatic Cl is best calculated from IV data (f bioavail is not a confounding factor) Cl cannot exceed the cardiac output of the animal

36 Clearance Mechanisms Top 200 Drugs Prescribed in US Route of Clearance of Top 200 Prescribed Drugs Metabolism Renal Bile Metabolite assessment in vitro & in vivo aids in identifying enzymes involved in clearance UGT Esterase P450 CYP UGT esterase FM NAT MA J. A. Williams et al Drug Metab. Dispos. 32, A4 1A2 2D6 2C9 2C19 More recent analyses consistent with these results CYP1A1 CYP1A2 CYP2B6 CYP2C9 CYP2C19 CYP2D6 CYP2E1 CYP3A

37 rigin of the Term Cytochrome P450 Ryo Sato Heme iron in P450 is usually in the ferric (Fe +3 ) state When reduced to the ferrous state (Fe +2 ), P450 can bind ligands ( 2, C). The complex between ferrous P450 and C absorbs light maximally at 450 nm Cytochrome P450 derives its name from this peak absorbance mura and Sato (1962) J. Biol. Chem. 237,

38 P450s & Substrates SUBSTRATES polymorphisms in humans can impact PK of drugs PM = poor metabolizer EM = extensive metabolizer IM = intermediate metabolizer

39 Aliphatic Hydroxylation HN N H HN N H S S CYP2C9 Tolbutamide CH 3 CH 2 H H -1 hydroxylation Lauric acid 7 CH H CYP4A H 7 7 CH CH H hydroxylation CYP3A4 Testosterone H

40 Aromatic Hydroxylation Cl N H CYP2E1 H Cl N H Chlorzoxazole CYP2A6 H Coumarin C 2 H 5 CH 3 N H CYP2C19 C 2 H 5 CH 3 N H (S)-Mephenytoin H

41 Mammalian Conjugation Conjugate Coenzyme Form 3'-Phosphoadenosine-5'-phosphosulfate (PAP S) Grou ps Co njug ated UDP - Glucuron osyltransferase Uridine-5'-diphospho- -D-glucuronic acid (UD PGA) CH H H NH -H, -CH, -NH 2, Glucuronide H -NR 2, -SH, C-H H N P P H H H H Transferase En zy me NH 2 & Sulfate H S N P N H H 2 3 P H N N -H, -NH 2 Su lfo transferase Potential for sustained exposure Glycine and glutamine Glutathione Activated acyl or aroyl coenzyme A cosubstrate R (Ar) HS SCoA + H 2 N H CH R Glutathione (GSH) H NH 2 N CH H H -CH Ar-X, arene oxide, epoxide, carbo catio n or related Glycine N-acyltransferase Glutamin e N-acyltransferase Glutath io ne S-transferase N H CH Acetyl Acetyl coenzyme A H 3 C SCoA -H, -NH 2 Acetyltransferase S-Adenosyl methionine (SAM) NH 2 Methyl HC H NH 2 CH 3 S + N N N N -H, -NH 2, -SH, heterocyclic N Meth yltransferase H H

42 Enterohepatic recirculation potential for drug-drug interaction and impact on PK (e.g., oral contraceptives and antibacterials)

43 HALF-LIFE (t 1/2 ) The time it takes for the amount or concentration to fall to ½ its original value For 1 st order processes: t 1/2 = 0.693/k PK Half life is a dependent parameter and depends on Cl and V V t 1/ * Cl Half life helps to determine time to steady state in an infusion or repeat dosing Helps to select the dosing interval in multiple dosing

44 Contributing Factors to Dosing for orally dosed small molecules Volume of Distribution Clearance Absorption Duration of exposure needed Half-life PK/PD Bioavailability Efficacious Concentration HW FTEN HW MUCH Reflection of ral Clearance (CL/F) Adapted from bach RS, Current pinion in Drug Discovery and Development (1):36-44.

45 CNCN (µm) Multiple Dosing TXICKINETICS half-lifes to steady state ACCUMULATIN/SATURATIN If AUC DAY n /AUC DAY 1 = 1 then N ACCUMULATIN R AT SS STEADY STATE If AUC DAY n /AUC DAY 1 < 1 then INDUCTIN (adaptive response P450 levels increased increased metabolism) 0.5 INDUCTIN TIME (hours) If AUC DAY n /AUC DAY 1 > 1 then ACCUMULATIN/CLp SATURATED

46 Test a pharmacology hypothesis garden variety mouse model improved exposure poor exposure target level for efficacy by improving the exposure of your test article with a P450 inactivator

47 Resolving exposure issues in rodent PC (leveraging a pan-p450 inactivator) 1-aminobenzotriazole (ABT) mglur4 positive allosteric modulator ++ ng*hr/ml 1-aminobenzotriazole (ABT) pan P450 inactivator ++ AUC = AUC = 40 Cytochrome P450: Structure, Mechanism and Biochemistry, 3 rd Ed. P.. demontellano

48 Contributing Factors to Dosing for orally dosed small molecules Volume of Distribution Clearance Absorption Half-life Bioavailability Duration of exposure needed PK/PD Efficacious Concentration HW FTEN HW MUCH Adapted from bach RS, Current pinion in Drug Discovery and Development (1):36-44.