Medium Throughput pK a Determinations of Drugs and Chemicals by Reverse Phase HPLC with an Organic Gradient

Automated techniques in pKa determination: Low, medium and high-throughput screening methods

Drug discovery programs that generate hundreds of new molecular entities need efficient methodologies for physicochemical profiling. Several high-throughput methods for pK_a screening have been developed in the last 15 years to determine this key physicochemical parameter. Separation techniques such as HPLC-MS or capillary electrophoresis are particularly well-suited due to their high throughput and capacity to deal with impure or complex samples. In addition, potentiometric and (mostly) UV-metric-based methods (plate-based and automated systems), find their place as very precise methodologies for pK_a determination despite of somewhat lower throughput. Finally, pK_a prediction software packages are useful estimator tools but, to date, they cannot replace experimental measurements when accurate pK_a values are required.

RP-HPLC determination of dissociation constant using solely aqueous mobile phase

The proposed HPLC method using solely or nearly 100% aqueous mobile buffer as mobile phase offers fast determination of dissociation constant for compounds in relatively wide range of lipophilicity (log P from -2.26 to 2.26). The dissociation constant value for simpler chemical compounds can be determined via only 8 chromatographic runs. The number of needed chromatographic separations depends on the structural complexity of the tested compound. Moreover, the proposed method does not require a measurement of Yasuda-Shedlovsky extrapolation that includes several pK_a determinations in solutions with different methanol content which speeds up considerably the procedure. The methodology is suitable for evaluation of large series of drug candidates, which can be present as complex mixtures and in small amounts.

Iontophoresis-targeted, follicular delivery of minoxidil sulfate for the treatment of alopecia

Although minoxidil (MX) is a drug known to stimulate hair growth, the treatment of androgenic alopecia could be improved by delivery strategies that would favor drug accumulation into the hair follicles. This work investigated in vitro the potential of iontophoresis to achieve this objective using MX sulfate (MXS), a more water-soluble derivative of MX. Passive delivery of MXS was first determined from an ethanol-water solution and from a thermosensitive gel. The latter formulation resulted in greater accumulation of MXS in the stratum corneum (skin's outermost layer) and hair follicles and an overall decrease in absorption through the skin. Anodal iontophoresis of MXS from the same gel formulation was then investigated at pH 3.5 and pH 5.5. Compared with passive delivery, iontophoresis increased the amount of drug reaching the follicular infundibula from 120 to 600 ng per follicle. In addition, drug recovery from follicular casts was threefold higher following iontophoresis at pH 5.5 compared with that at pH 3.5. Preliminary in vivo experiments in rats confirmed that iontophoretic delivery of MXS facilitated drug accumulation in hair follicles. Overall, therefore, iontophoresis successfully and significantly enhanced follicular delivery of MX suggesting a useful opportunity for the improved treatment of alopecia.

Solubility advantage of amorphous pharmaceuticals: II. Application of quantitative thermodynamic relationships for prediction of solubility enhancement in structurally diverse insoluble pharmaceuticals

PURPOSE

To quantitatively assess the solubility advantage of amorphous forms of nine insoluble drugs with a wide range of physico-chemical properties utilizing a previously reported thermodynamic approach.

METHODS

Thermal properties of amorphous and crystalline forms of drugs were measured using modulated differential calorimetry. Equilibrium moisture sorption uptake by amorphous drugs was measured by a gravimetric moisture sorption analyzer, and ionization constants were determined from the pH-solubility profiles. Solubilities of crystalline and amorphous forms of drugs were measured in de-ionized water at 25°C. Polarized microscopy was used to provide qualitative information about the crystallization of amorphous drug in solution during solubility measurement.

RESULT

For three out the nine compounds, the estimated solubility based on thermodynamic considerations was within two-fold of the experimental measurement. For one compound, estimated solubility enhancement was lower than experimental value, likely due to extensive ionization in solution and hence its sensitivity to error in pKa measurement. For the remaining five compounds, estimated solubility was about 4- to 53-fold higher than experimental results. In all cases where the theoretical solubility estimates were significantly higher, it was observed that the amorphous drug crystallized rapidly during the experimental determination of solubility, thus preventing an accurate experimental assessment of solubility advantage.

CONCLUSION

It has been demonstrated that the theoretical approach does provide an accurate estimate of the maximum solubility enhancement by an amorphous drug relative to its crystalline form for structurally diverse insoluble drugs when recrystallization during dissolution is minimal.