(M1430-04-23) Evaluating pH and Buffer Effects on (-)- and (+)- Phenserine Chemical Stability

Purpose: (-)-Phenserine, a selective inhibitor of acetyl cholinesterase (AChE), is a potential new therapy in the treatment of Alzheimer’s Disease (AD), and posiphen ((+)-phenserine), although associated with only limited AChE inhibitory activity, has been shown to reduce the formation of amyloid precursor protein (APP), tau, and α-synuclein, dysregulated proteins associated with neurodegenerative diseases including Alzheimer’s and Parkinson’s disease. Phenserine is a phenyl-carbamate analog of physostigmine which has been reported to show poor chemical stability in near neutral and alkaline pH environments. The objective of this work was to investigate pH and buffer effects on the stability of (-)-phenserine and posiphen ((+)-phenserine) for a more detailed understanding of their impacts on phenserine chemical stability under various environmental conditions.

Methods: The stability of posiphen over a 12 hr interval was evaluated between pH 2-7 at 21°C and 40°C. Buffer effects were examined at pH 4, 5, and 7 using acetate, citrate, and phosphate buffers to identify any direct buffer effects on the degradation of the drug. First order plots were used to determine k_obs(app), and activation energies at pH 7 and 8 were determined from the temperature studies. Theoretical k_37°C values at pH 2-7 were also estimated. The stability of phenserine at pH 4 and 7 was evaluated at 40°C using citrate and phosphate buffers, respectively, to confirm that posiphen and phenserine possess similar chemical stabilities in aqueous environments.

Results: Posiphen showed good stability between pH 2-5 with no significant degradation of the drug over a 12 hr period. Posiphen degraded rapidly at pH 7 and 8 with only 31.6% and 6.1%, respectively, of the drug remaining in solution after 10 hr at 40°C. The predicted k_37°C at pH 7 was 120x10^-5 min^-1 resulting in an estimated half-life based on the appearance of the final degradation product of 9.6 hr at near-physiologic pH. As expected, the stability of (-)-phenserine was nearly identical to that observed for posiphen.

Conclusion: Similar to other physostigmine analogs, both phenserine, and posiphen degrade to form a pink/reddish-brown product. Both enantiomers show good stability in acidic pH environments but show very rapid degradation above pH 7. There were no significant changes in the degradation rate constants when acetate, citrate, or phosphate buffers were used at a single pH, suggesting no direct buffer effects on the degradation rate. Careful selection of the pH of the formulation environment can enhance the stability and shelf-life of phenserine, a promising new therapeutic agent against Alzheimer’s Disease.

Acknowledgments: The work was funded by NIH 75N95D20P00146