Methods for Assessing Bioequivalence of Topical Products: How should FDA Redirect its Research Program? Ajaz Hussain, Ph.D. Director (Act.), Office of Testing and Research OPS, CDER, FDA 17 November 2000
Bioavailability of Topical Drugs • Factors that effect bioavailability • Drug attributes (solubility and dissolution rate in the vehicle, size, charge, membrane permeability and metabolism) • Vehicle attributes (drug solubility and dissolution rate, spreading-ability, adhesion, ability to alter membrane permeability) • Membrane attributes (status of barrier function, exudates, blood flow, metabolic capacity,..) • Method of application
Bioequivalence of Topical Products • Equivalent rate and extent of exposure at the intended “site(s) of action” • Equivalent rates of membrane penetration and permeation • function of vehicle effects on these processes • function of rate of drug release from the vehicle • Equivalent application site - formulation contact time and area • [Equivalent systemic exposure]
How should FDA Redirect its Research Program? • Current research projects • DPK projects • [Topical (Vaginal) Microbicide Products] • Proposed research projects • Body of evidence need for regulatory acceptance of DPK approach? • Other tests to complement DPK? • New methods for bioequivalence assessment?
Current DPK Research Activities • DPK Study at University of Utah - Tretinoin “reference” product plus two test products (based on clinical evidence 1 equivalent to RP and 1 inequivalent to RP) • FDA Investigators- Surendra Shrivastava and Don Hare • Intramural DPK Study - Reproducibility of Utah Study plus other “method” issues • PI’s Robbe Lyon, Tapash Ghosh, Mamta Gokhle, Martin Okun • Near-IR study - PI: Everette Jefferson • If both studies are “positive,” would this evidence be sufficient to introduce DPK in regulatory practice? • Yes • No
DPK Approach for Bioequivalence: Concerns • Stratum corneum skin • Can not be derived from first principles • Generalization of collected empirical evidence? • Clinical relevance? • DPK will not provide accurate estimates of drug bioavailability under certain disease conditions and for other routes of administration (e.g., vaginal products)
Key Questions • Can comparable DPK profiles be used to assesses bioequivalence between two (pharmaceutical equivalent) products? • Equivalent SC exposure (SCT/SCR) = equivalent follicular exposure (FT/FR)? • Equivalent SC (healthy) exposure = equivalent exposure in disease states? • Does [Q1 + Q2] criteria ensure equivalent physical attributes for multi-phasic systems? • Increases the divide between innovator and generic firms • Management issues
Rephrasing the Concerns with DPK • Two topical products applied to skin surface provide equivalent rate and extent of drug exposure in all layers of the skin when these products exhibit equivalent • thermodynamic activity of drug in vehicle • interfacial transport kinetics • SC Vs. follicles? • effect of excipients on skin permeability • healthy Vs. disease? • skin contact time and area • healthy Vs. disease?
Role of follicular transport on BE assessment? • Equivalent SC exposure (SCT/SCR)= equivalent follicular exposure (FT/FR)? • Likely when drug is in solution (single phase system)? • Equivalent SC exposure Equivalent (thermodynamic activity + excipient effects on SC) • Higher potential for differences when drug is encapsulated or suspended (particle size differences) and/or multi-phase system? • Retin-A Micro - acrylate copolymer porous microspheres • “contribution to decreased irritancy by Microsponge system has not been established.”
Role of follicular transport • Possible to modulate follicular transport (iontophoresis or low intensity ultrasound) - a approach to challenge DPK?
Supporting evidence can be generated via in vitro experiments using excised human skin different anatomical sites possible to maintain viability (~ 24 hr) emulate compromised SC barrier functions? Indirect supporting evidence via transport and skin distribution studies Direct supporting evidence via visualization of follicular and nonfollicular transport laser scanning confocal microscopy Mechanistic evidence plus distribution and imaging approaches?
Body of Evidence? • Empirical evidence • DPK Vs. Clinical Studies • Proof of concept for the products evaluated • Generalization of empirical evidence? • Mechanistic basis (“Reductioinst” approach) • ----------------------------------------------------- • New methods • Complementary or stand-alone
Vaginal Products • The following slides provide a brief summary of current research on topical microbicide vaginal products • this research has a broader scope than bioequivalence • is an example of the “reductionist” approach • linking physics with physiology to identify critical product attributes and explain how these attributes effect product performance
Desired Distribution Profile ofCertain Vaginal Formulations CERVIX VAGINA MUCUS INTROITUS PROPHYLACTIC COATING CONTRACEPTIVE COATING
Interactionsin the Vagina Fluid Contents Anatomy, Geometry Microbicide Formulation Mechanical Properties Surface Properties
“SQUEEZING” visceral contractions pressure tissue elasticity rugae mucus, transudate GEL “SEEPING” surface energies interfacial tensions Pre-Coital Forces Acting on a Bolus of Gel in Vagina “SLIDING” gravity David Katz. Duke University
THEORY • SIMULATION vehicle epithelial surfaces F R FORMULATION 2h Solution for elastic surfaces (E, n); lubrication approximation; power law fluid (n, m); conserved bolus volume V = 2hoπ R2 Area(t)= David Katz. Duke University Mechanistic Analysis of Sub-processes (Squeezing)
SLIDING THEORY Vavg velocity depends upon… vagina properties tilt anglea tissue separationHtot gel properties density rheology Htot velocity profile a David Katz. Duke University
Viscosity vs. Shear Rate Gynol II KY Plus Conceptrol Advantage-S David Katz. Duke University
Vaginal Gel Thickness Distribution (Advantage) 4000 3000 2000 1000 Depth of Coating (mm) 125 introitus 100 75 0 45 50 90 Axial Dist. (mm) 135 180 25 Azimuthal Angle (deg.) 225 270 315 David Katz. Duke University
4000 3000 2000 1000 Depth of Coating (mm) 125 100 introitus 75 0 45 50 90 Axial Dist. (mm) 135 180 25 Azimuthal Angle (deg.) 225 270 315 Vaginal Gel Thickness Distribution (Conceptrol) David Katz. Duke University
David Katz. Duke University Axial and Angular Dependenceof Coating Thickness Distribution Conceptrol Advantage Scaled to length of 71.11 mm Scaled to length of 51.67 mm % Volume Distal to Fornix 39% 85% % of Area Coated 36% 100%