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PharmLabTM: A Computer Program for the Calculation and Visualization of Drug Degradation pH Rate Profiles (Galley)

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Michael B. Bolger and Ian S. Haworth


American Journal of Pharmaceutical Education

Vol. 61, No. 3 (1997)
DOI: aj6103281.pdf




American Association of Colleges of Pharmacy


PharmLab™ is an MS-Windows program that computes a time course for drug degradation and can be used to calculate a pH rate profile for a drug. We have used the program in an educational setting to illustrate and examine the kinetics of the hydrolysis of ionizable esters as a function of the molecular structure of the ester. In addition to the educational value, we believe that the simple relationships used to parameterize the program provide some new insights into the pH rate profile for ester hydrolysis, and could offer an accurate predictive method, given a broader-based parameterization. The program is based on experimental rate profiles and on numerical integration of the differential equations that describe the hydrolysis of ionizable esters. The influence of the pKa of the ionizable group on the experimental rate profiles follows from standard differential rate equations. Empirical parameters derived as a function of the pKa values of the reactants and products in the hydrolysis reaction were used to calculate changes in rate constants caused by changes in the molecular structure. We illustrate the derivation of these parameters for a series of esters structurally similar to procaine (2-(N,N-diethylamino)ethyl 4-aminobenzoate), and show how the pH rate profiles we obtain can be interpreted in terms of the structural differences between the esters. The PharmLab™ program 1.0 will be made available on the USC School of Pharmacy homepage on the World Wide Web.

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