Dosage Determination from Preclinical to Proof-of-Concept Trials, (Including Toxicology)

1. 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 vivounbound plasma drug concentration or daily drug exposure to help choose human efficacy DOSES

4. Drug Development Failures – UK ’64-’85 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 mg•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. Cmax = 10.8 mg/mL AUC(0-24) = 136 mg•hr/mL 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 Dose = 50 mg/kg/day

10. Measured Drug Exposure in Rat Tox Studies

11. Animal Toxicology: Human Exposure Limits Are Set by Daily Drug Exposure (AUC0-24) RAT DOG MOUSE No-Effect Dose Limit: 200 mg•hr/mL – determines maximum permissible human exposure

12. PK Modeling of Drug in Human Plasma (daily dosing of 50 mg oral) Human Cmax Limit based on Animal Toxicology (Max no-effect dose AUC0-24) (mg/mL) AUC0-24

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 • Napiergram: graphic comparison of log10unbound plasma drug concentrations associated with pharmacology and with safety concerns John Napier (1550-1617)

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 Cmax = 2,500 nM or 0.5 mg/mL (unbound) Cmin = 125 nM or 0.026 mg/mL (unbound)

19. PK Modeling of Drug in Human Plasma (daily dosing of 50 mg oral) (mg/mL) Animal Adverse Limit (EC50 for ataxia) Cmax (hi dose) Actual Human PK – mid dose Cmax (mid dose) Cmax (low dose) EC20 for Efficacy in Animal Model

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!

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