Preformulation and formulation studies of RH1: A new investigational anti-tumor agent

Abstract

Currently, the National Cancer Institute is investigating RH1 for its potential use as an anti-tumor agent. A parenteral formulation is desired but the drug is highly unstable in aqueous solutions. Various effects on the stability of RH1 are investigated. All the reactions of RH1 follow first-order kinetics. The maximum shelf-life, obtained in neutral conditions, is about one week. The pH-rate profile shows slopes of approximately -1 in acidic conditions and +1 in basic conditions indicating that the degradation of RH1 is specific acid-base catalyzed. The energies of activation at pH's 6.0 and 7.0, and 8.0 confirm a difference in the mechanisms of decomposition in acid and base. No significant effect of light or anti-oxidants is seen in either acidic or basic solutions, however, the chelating agent (EDTA) significantly slows the degradation of RH1. Use of pure ethanol increased the stability four times that over the buffered solution at pH 7.33 resulting in a shelf-life of approximately one month. Four major degradation products are observed in acid pH. The results of LC-MS suggests that the degradation of RH1 involves hydrolysis of the aziridine rings to form the mono-ethanolamine derivative (degradation product I). The second reaction involves nucleophilic substitution of the aziridine ring(s) by hydroxyl groups to form mono-hydroxy derivatives. Depending upon which aziridine ring is hydrolyzed, two additional degradation product (II and III) can be formed. Two isomers are possible for the three proposed decomposition products. The absence of any degradation products with molecular masses expected from the reaction of both the aziridines suggests that one of the isomers is preferentially formed. This is most likely due to the fact that the C2 aziridine nitrogen can form a hydrogen bond with the neighboring methyl hyroxy group, making it less reactive. A freeze-dried formulation buffered at pH 7.0 containing 5% hydroxypropyl-beta-cyclodextrin that can be reconstituted with a buffer (pH 7.0) is recommended for parenteral use. A prodrug, 2,5-diaziridinyl-3,6-dihydroxymethyl-1,4-benzoquinone, is proposed based upon the fact that two hydrogen bonds between the hydroxymethyl groups and aziridine nitrogens will be possible that may lead to improved stability of the drug.