Dosage Determination from Preclinical to Proof-of-Concept Trials,

(Including Toxicology)

Charlie Taylor, PhD

CpTaylor Consulting

Chelsea, MI, USA

2

Choosing Doses for POC:

• Preclinical and early studies that enable dose selection

• Reasons for drug failure in clinical phase 2-3

• Need to choose both low (ineffective) and high (maximum tolerated) doses within dose range

• Biomarkers (one endpoint: animal → human translation)

• PK/PD modeling – EC50 as a target for efficacy or AEs

• Toxicology/toxicokinetics – daily AUC(0-24) as a limit

• Putting it together – visualizing multiple datasets

• Human population PK modeling – determine which doses best fit the constraints

3

Sequence of Studies Needed Prior to Clinical Proof-of-Concept

• Preclinical in vitro studies: action at drug target (pharmacology)

• Preclinical in vivo pain models: indicate treatment of disease

• Safety pharmacology: animal studies for adverse effects

• Preclinical (& human liver microsome) metabolism studies

• Animal toxicology & toxicokinetic studies to identify safety issues

• Clinical Phase I studies of pharmacokinetics and tolerance in healthy human volunteers

• (Optional) Biomarker studies with both animal models and humans to establish proof of pharmacology in vivo (apart from efficacy)

***

*

*

* Requires in vivo unbound plasma drug concentration or dailydrug exposure to help choose human efficacy DOSES

*

4

Drug Development Failures – UK ’64-’85

Efficacy29%

Toxicol.11%

PharmKin39%

Adv.Events10%

Other

Prentis et al. (1988) Brit J Clin Pharmacol 25:387-396

5

Determine Initial Phase 2 Dose Range ??

• Clinical doses MUST encompass both low end (lack of efficacy) and high end (maximum tolerance)

• Data from animal efficacy, animal safety, biomarker and human tolerance ALL must be considered

• The peak unbound plasma drug concentration (animal studies), daily exposure (AUC0-24 - tox) and human multiple-dose PK each need consideration

• How to consider all these factors??

6

One Approach: Biomarkers – Surrogate HUMAN Endpoints for Efficacy

• Defines drug action in vivo

• Examples: Imaging, Adverse Event or Mechanism

• e.g. PET to measure receptor occupancy in CNS

• e.g. Nystagmus, dizziness, balance platform

• e.g. Experimental pain model w/ volunteers

• e.g. Electrographic response (EEG, retinogram, TMS)

Biomarkers Allow no-go decision prior to proof of efficacy, for example:

• Poor oral drug absorption or lack of CNS penetration

• Lack of receptor occupancy at highest safe dose

7

Hypothetical Human Biomarker:

• Criterion: 75% drug receptor occupancy in human brain @ high dose

• This criterion met at animal effective dose (animal PET study)

• Drug displacement of PET ligand in human brain: 18F-x-drugamine given IV in tracer amount

• If greatest human volunteer dose of experimental drug reaches < 30% occupancy, NO-GO

• If greatest human volunteer dose > 75% occupancy, GO (further development)

• Caveat: Criteria must be selected based on results with a prior known compound – Otherwise, risk of poor validation

8

Toxicology Findings (non-pharmacology) are Based on Daily Drug Exposure (AUC0-24)

• Repeated-dose animal tox studies determine lowest toxic dose and greatest no-effect (daily) dose

• Toxicokinetics determine drug exposure (AUC0-24) in g•hr/mL at greatest no-effect dose

• e.g. Drug X has 8 hr half-life; Cmax and AUC are determined from plasma drug samples taken 0, 1, 4, 7, 12, and 24 hr after single oral dose at steady-state

• Similar human pharmacokinetic data and PK modeling determine human drug exposure (AUC0-24) @ doses

• Analysis is adjusted for different drug binding of plasma proteins between species

9

Calculation of Animal Drug Exposure - Toxicokinetic AUC(0-24)

• Samples of drug in plasma of animal tox species — Begin sampling after reaching repeated dose steady state— Orange symbols are mean from n = 8

• Mathematical fit to curves of oral absorption & elimination• Measure area under curve for 0-24 hr = Drug Exposure

Cmax = 10.8 g/mL

Hr After Dose

0 4 8 12 16 20 24Fre

e P

lasm

a D

rug

Co

nc. (

g/m

L)

0

2

4

6

8

10

12

14

AUC(0-24) = 136 g•hr/mLDose =

50 mg/kg/day

10

Measured Drug Exposure in Rat Tox Studies

Toxicokinetic Parameters in Multidose Oral Toxicity Studies

Species Durat.Dose

(mg/kg)

Plasma Concentration (g/mL)

AUC(0‑24) (g·hr/mL)

Male Female Male Female

Rat 2 Week

Gavage50012502500Diet50012502500

13.527.647.4

11.525.950.7

9.9225.240.7

10.719.032.4

120.0332.0626.0

199.0491.0921.0

102.0334.0602.0

181.0336.0606.0

11

Animal Toxicology: Human Exposure Limits Are Set by Daily Drug Exposure (AUC0-24)

AU

C(0

-24)

100

1000

10000

100000

RAT

DOG

MOUSE

No-Effect Dose Limit:200 g•hr/mL – determines maximum permissible human exposure

12

PK Modeling of Drug in Human Plasma (daily dosing of 50 mg oral)

Time (hours)

0 10 20 30 40 50 60 70 80 90 100 110 120

Fre

e P

lasm

a D

rug

Con

c. (

nM)

1

10

100

Human Cmax Limit based on Animal Toxicology(Max no-effect dose AUC0-24)

AUC0-24

g/

mL)

13

CYP2D6 Heterogeneity – Ca2+ Channel Blocker

Smith & Jones (1999) Curr Opin Drug Discov Devl 2:33-41

14

Q: How to Predict Human Efficacious

and Adverse Drug Doses Based on

Animal Efficacy, Animal AEs and

Human Pharmacokinetic Data??

A: Compare plasma drug Cmax

obtained in animal pharmacology

tests using a Napiergram to human

Pharmacokinetic Cmax data

15

“Napiergram”• Named for John Napier of Merchistoun (aka Marvelous Merchiston, Scotland)

• Inventor of Napier’s bones (slide rule), popularization of logarithms and the decimal point

• Also: used a pet black rooster to tell fortunes and devine truths

John Napier (1550-1617)

• Napiergram: graphic comparison of log10 unbound plasma drug concentrations associated with pharmacology and with safety concerns

16

From Dose:Response experiments: Obtain ED5, ED50, ED95

17

Transform Pharmacology from ED50 to EC50

18

Napiergram: Many Pharmacology Datasets – Animal Cmax for doses with 5% 50% & 95% effect

Fre

e F

rac

tio

n (

nM

)

1

10

100

1000

10000

100000

1000000

Cmax = 2,500 nM or 0.5 g/mL (unbound)

Cmin = 125 nM or 0.026 g/mL (unbound)

19

PK Modeling of Drug in Human Plasma (daily dosing of 50 mg oral)

Time (hours)

0 10 20 30 40 50 60 70 80 90 100 110 120

Fre

e P

lasm

a D

rug

Con

c. (

nM)

1

10

100

Animal AdverseLimit (EC50 for ataxia)

EC20 for Efficacy in Animal Model

Cmax (hi dose)

Cmax (mid dose)

Cmax (low dose)

Actual Human PK – mid dose

g/

mL)

20

Phase 2 Dose Selection (final chapter)

• Requires Deliberation from team of experts:

• Animal tox, Pharmacokinetics, PK/PD modeling, Clinical research, Preclinical pharmacology, (Biomarkers)

• Who pays the clinical trial bills? Clinical Research

— Despite planning, dosage and regimen often are readjusted during Phase 2 (toleration, efficacy or new safety findings)

— Dosages MUST continue to include both low (ineffective) and maximal tolerated dosages to provide basis for FDA approval

— Dose toleration may vary between healthy volunteers and patients with serious disease

21

SUMMARY: Preclinical Studies to Determine Phase 2 Dose Selection

• In vitro and in vivo animal pharmacology – target Cmax for therapy and adverse effects

• Animal toxicology & toxicokinetic studies – determines maximal human drug exposure (AUC0-24)

• Phase 1 Clinical trials- determines human pharmacokinetics & drug exposure

• Napiergram – allows consideration of Cmax from multiple animal datasets & compare to human PK

• Phase 2 dose adjustment is common!

22

23

Example “Drug Killer” Problems

• Poor Oral Absorption (F < 25%)

• Poor Aqueous Solubility

• Poor Elimination Kinetics (t1/2 < 4 hr or t1/2 > 36 hr)

• Nonlinear Elimination Kinetics (e.g. blocked clearance at high doses)

• Extensive metabolism to active or toxic compound

• Excessive plasma protein binding (> 99%)

• Metabolism by variable CyP450 (CYP2D6, CYP2C19)

• Cardiac Q-T interval prolongation (hERG channel block)

• Genotoxic compound (Ames positive)

• Hepatic toxicity

PK

Tox