As. Kearney et al., PREFORMULATION STUDIES TO AID IN THE DEVELOPMENT OF A READY-TO-USE INJECTABLE SOLUTION OF THE ANTITUMOR AGENT, TOPOTECAN, International journal of pharmaceutics, 127(2), 1996, pp. 229-237
Topotecan ((S)-9-dimethylaminomethyl-10-hydroxycamptothecin hydrochlor
ide; SKF-S-104864-A) is a promising anticancer agent that is currently
available as a lyophilized formulation. To evaluate the feasibility o
f formulating topotecan as a ready-to-use injectable solution, the pH-
solubility profile was generated over a pH range of 2.5 to 4.5 at 25 d
egrees C, and the pH-stability profile was generated over a pH range o
f 2 to 4, a temperature range of 60-80 degrees C; and at an ionic stre
ngth of similar to 0.15 M. The former experiments revealed that desira
ble solubilities (i.e. 2.5 mg/ml in free base equivalents) are achieva
ble at pH < 4, whereas extrapolation of the degradation kinetics to 25
degrees C indicated that desirable stabilities (i.e. less than or equ
al to 2% degradation of topotecan over 2 years) are achievable at pH <
3. The stability results are consistent with a degradation mechanism
involving deamination of topotecan proceeding through a reactive quino
ne methide intermediate. Subsequent attack of water on this intermedia
te gives the dihydroxylated product, 9-hydroxymethyl-10-hydroxycamptot
hecin, which may then lose formaldehyde to form the monohydroxylated p
roduct, 10-hydroxycamptothecin. The overall study results suggest that
a solution pH of less than or equal to 2.5 is most appropriate for th
e formulation of a ready-to-use solution. Prototype formulations meeti
ng these criteria have been placed on long-term stability.