Determination of the molecular complexation constant between alprostadil and alpha-cyclodextrin by conductometry: implications for a freeze-dried formulation

The binding constant between alprostadil (PGE₁) and -cyclodextrin (-CD) was determined at four temperatures using conductance measurements. Alpha-cyclodextrin is an excipient material in Caverject dual chamber syringe (DCS) that was added to enhance stability. The binding constant was used to calculate the amount of PGE₁ free upon reconstitution and injection, since only the free drug is clinically active. The conductivity measurement is based on a decrease in specific conductance as alprostadil is titrated with -CD. The change in conductivity was plotted versus free ligand concentration (-CD) to generate a binding curve. As the value of the binding constant proved to be dependent on substrate concentration, it is really a pseudo binding constant. A value of 742 ± 60 M⁻¹ was obtained for a 0.5 mM solution of alprostadil at 27 °C and a value of 550 ± 52 M⁻¹ at 37 °C. These results compare favorably to values previously obtained by NMR and capillary electrophoresis. Calculation of the fraction PGE₁ free upon reconstitution and injection show it to approach the desired outcome of one. Hence, the amount of drug delivered by Caverject DCS is nominally equivalent to that delivered by Caverject S. Po., a predecessor product that contains no alpha-cyclodextrin.