When a NCE arrives from a synthesis lab, it often is accompanied by a chromatogram to demonstrate its purity. The chromatographic parameters are generally a C18 column with a 0 to 100% acetonitrile gradient over 15 minutes. While conceivably suitable for a drug substance, this is not that helpful for developing an assay for the drug product.
The first step in creating a semisolid formulation is determining the solubility of the API in a wide range of liquids, from water to mineral oil. The analysis of these samples is derived from the acetonitrile gradient; if the compound elutes at high organic content, the initial conditions are set at 75% aqueous phase/25% acetonitrile with a slower gradient to allow for resolution of any closely related impurity.
Analysis of stressed samples at high temperature demonstrates which excipients are incompatible with the API. The peak shape is carefully examined for the presence of closely eluting degradants. If indicated, adjustments to the gradient or investigations of other stationary phases to achieve resolution are made.
Whether the API is dissolved or suspended, in a gel, emulsion or ointment, all affect the parameters of the analytical procedure. Must the API be extracted from the matrix or can we “dilute and shoot”? Does the chromatography of the compatibility study resolve the peaks of interest from all excipients? Is the prepared sample solution stable or do compounds continue to degrade? All these questions must be answered so that the results generated can be relied on to make a decision for selection of a lead formulation.
Once a lead formulation is chosen, the FDA has issued a guidance to help in the preparation of phase 1 trial material[1]. The analytical procedure continues to be refined. Is the sample preparation robust and efficient enough for the analysis of large sample sets from stability studies? Does the gradient wash off material injected with the sample or does performance degrade with repeated injections? Will this method allow for the determination of functional excipients or are additional methods required to characterize the product? Is the method suitable for validation? Are new peaks observed during the stability studies of the product? Does scale up affect the method performance? As the product matures, the method can be adapted to provide the data to accurately characterize the product.
[1] FDA Guidance for Industry. CGMP for Phase 1 Investigational Drugs, July 2008 https://www.fda.gov/regulatory-information/search-fda-guidance-documents/current-good-manufacturing-practice-phase-1-investigational-drugs