1. Charlie Taylor, PhD CpTaylor Consulting Chelsea, MI, USA
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 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:

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 (AUC 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 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 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 mg/mL
AUC(0-24) =
136 mg•hr/mL
Dose =
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
Rat
2 Week
Gavage
500
1250
2500
Diet
13.5
27.6
47.4
11.5
25.9
50.7
9.92
25.2
40.7
10.7
19.0
32.4
120.0
332.0
626.0
199.0
491.0
921.0
102.0
334.0
602.0
181.0
336.0
606.0

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 log10 unbound plasma drug concentrations associated with pharmacology and with safety concerns
John Napier ( )

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 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)
Cmax (mid dose)
Actual Human
PK – 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