Dear all,
The oral dosage of the API is known (3mg/kg/day) by oral suspension. There is no standard or marketed topical/dermal product of that API for correlation. How do I select and formulate the dosage strength (%w/w) of an API in final topical emulgel?
There is no universal and constant relationship between the oral and topical doses for the same drug. Usually, it between 20-30% higher for the topicals owing to many factors of drugs pharmacokinetic, physico-chemical properties of the drug and the formulations (both oral and topical formulations), and the skin permeability factors and parameters as well as multiple other factors (TOPICAL DELIVERY - The Importance of the Right Formulation in Topical Drug Development (drug-dev.com). Some of the following info is attached for generalized concepts:
DOI: 10.1345/aph.1L128
Quality Assessment of API in Semisolid Topical Drug Products | Request PDF (researchgate.net)
It depends on the purpose of the drug, targeting skin or for systematic effect. If it is for systematic effect, you can use drug concentration in plasma as a reference. If it is to target skin, you need to estimate its local concentration. In either case, there is no direct correlation and you need to conduct studies to find out.
There is no conversion for deciding the topical dose. It depends on the following factors related to
a) Drug's physicochemical properties
b) Location, type, and area of skin for the administration of the drug
c) Conditions of the skin (normal or diseased)
d) Formulation characteristics and type of chemicals used
But for sure it would be higher than the oral dose if it is intended for systemic absorption.
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