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1.  Electronic and resonance effects on the lonization of structural analogues of efavirenz 
AAPS PharmSci  2001;3(4):26-29.
The solubility of 4 analogues of efavirenz was studied as a function of pH. The study evaluated the ionization behavior and determined the relative contribution of electronegative substituents versus resonance effects on the pKa value of the cyclic carbamate. The most profound lowering effect on the pKa was due to the presence of multiple electronegative substituents and in particular the trifluoromethyl and acetylene groups. The presence of chlorine on the benzoxazinone ring was found to have a slight impact on the pKa, although to a lesser extent. In the absence of any functional groups on the benzoxazinone ring system, the pKa shifted to a value of 13.2, which is 3 pH units above that of efavirenz and more closely correlates with typical literature values for cyclic carbamates.
doi:10.1208/ps030428
PMCID: PMC2751217  PMID: 12049491
pKa; Ionization; Benzoxazinone; Carbamate; Solubility; Efavirenz Analogs
2.  Physical chemical stability of warfarin sodium 
AAPS PharmSci  2001;3(1):18-25.
Crystalline warfarin sodium is an isopropanol clathrate containing 8.3% isopropyl alcohol (IPA) and 0.57% water upon receipt. The hygroscopicity and impact of moisture on IPA status as well as on the stability of the clathrate was studied at different relative humidities. The IPA loss and water uptake were simultaneous but they did not exchange at 1:1 molar ratio. At 58% relative humidity (RH) or below, the exchange process was insignificant. At 68% RH or above, the clathrate tended to lose IPA while absorbing water and reverting to the amorphous state. The rate of IPA loss and moisture uptake was a function of RH. The thermal stability of the crystalline warfarin sodium was also examined. Physical change occurred after isothermal storage for 24 hours at 80°C and 11 hours at 120°C. The rate of IPA loss was temperature dependent.
doi:10.1208/ps030103
PMCID: PMC2751235  PMID: 11741254
warfarin sodium; isopropyl alcohol; clathrate; amorphous; crystallinity; physical stability; thermal stability

Results 1-2 (2)