Pharmorphix^® Physico-Chemical Property Studies

It is well documented that physicochemical properties can be correlated to many ADME (Absorption, Distribution, Metabolism and Elimination) properties, particularly in terms of absorption.

It is therefore essential that as part of any lead-optimization program, physicochemical properties are optimized in parallel with activity and selectivity measurements to help identify potential drugs.

Many of these assays can utilize stock solutions in DMSO and sample requirements have been minimized by use of miniaturized assay conditions and sensitive detection methods such as HPLC equipped with UV Diode Array detectors.

Thermodynamic and Kinetic solubility can be measured in a wide range of buffers and formulation media with analysis by HPLC.

The measurement range is from approximately 0.0001 mg/ml (lower limit dependant on the UV extinction coefficient of the molecule) to >10mg/ml. The analysis also includes a measurement of the pH at equilibrium, as this is a key parameter when assessing the results.

Measurement of the intrinsic dissolution rate of materials to aid the selection of the best form of the API for progression. Examples of test materials include: Salts, Polymorphs, Amorphous materials, Hydrates, cocrystals and Formulations.

Dissolution rate of a material is a much better parameter for correlation with the likely in vivo performance of an oral formulation compared to thermodynamic solubility.

SAFC-Pharmorphix uses Sirius GlpKa instruments equipped with a D-PAS (UV) attachment to measure pKa values.

Normally the neutral form of the compound is insufficiently soluble in 100% aqueous conditions to enable the collection of enough data before the compound precipitates. Therefore measurements are typically made in methanol/water mixtures with a background electrolyte concentration of 0.15M KCl.

At least three titrations are made in different ratios of cosolvent/water and the data extrapolated back to 100% aqueous via a Yasuda-Shedlovsky plot. The pH range of the potentiometric experiment is approximately 3 to 11 using approximately 3mg of material per experiment.

Compounds which have a UV chromophore that is within approximately 3 bonds of the ionisable group can be measured by UV spectroscopy. Even by UV spectroscopy some compounds precipitate and again a cosolvent such as methanol can be used and the data sets extrapolated back to 100% aqueous via a Yasuda-Shedlovsky plot. The pH range of the UV method is approximately 1 to 12 using approximately 0.2mg of material per experiment.

At SAFC-Pharmorphix uses Sirius GlpKa instruments for the measurement of LogP and LogD profiles. These measurements are made after the pKa value(s) have been measured. We have two methods for measuring LogP/D values:

The compound is titrated in a similar way to the pKa determination in aqueous conditions except that 1-octanol is added to the solution. This causes a shift in the pKa value from which the LogP value is calculated. By performing additional experiments in different ratios of octanol to water the LogP value of the ionised form can be measured and an accurate LogD profile calculated.

A background of 0.15M KCl is maintained in these experiments which allows for ion pair partitioning. The pH range of the potentiometric experiment is approximately 3 to 11 using approximately 3mg of material per experiment.

This is carried out by partitioning a solution of the compound between a buffer and 1-octanol mixture. The concentration of the analyte in each layer is then quantified by HPLC analysis.

At SAFC-Pharmorphix the effect of temperature, pH, oxidation and light on the chemical stability of an active API can be studied with analysis by HPLC.

Storage of the material is performed over a series of saturated salt solutions at different temperatures to investigate the effect of temperature/humidity.