Activation and cytotoxicity of 2-alpha-aminoacyl prodrugs of methotrexate.
In an effort to improve the selectivity of the anticancer drug methotrexate (MTX), a series of potential prodrugs in which the 2-amino group was acylated with various alpha-amino acids (as well as L-pyroglutamic acid) was synthesized. Such derivatives are anticipated to be hydrolysed to MTX by appropriate aminopeptidases localized (over-expressed naturally or targeted as anti-tumor antibody conjugates) in the vicinity of the tumor. The L-leucyl, L-valyl, L-isoleucyl, D-alanyl and L-pyroglutamyl derivatives were assessed as to their suitability as prodrugs. Except for the L-pyroglutamyl compound, all derivatives decomposed slowly when incubated in phosphate buffer, pH 7.3; the formation of MTX was minimal. No major differences were observed when serum was included in the incubation medium, except for the L-leucyl compound, which was hydrolysed to MTX. The L-leucyl, L-valyl and L-isoleucyl derivatives were hydrolysed readily to MTX by aminopeptidase M (EC 3.4.11.2), while the L-pyroglutamyl and D-alanyl compounds were activated by pyroglutamate aminopeptidase (EC 3.4.19.3) (from Bacillus amyloliquefaciens) and D-aminopeptidase (from Ochrobactrum anthropi), respectively. When tested for inhibition of the target enzyme dihydrofolate reductase (DHFR; EC 1.5.1.3), 2-L-valyl-MTX showed inhibition two orders of magnitude poorer than that given by MTX, in agreement with the expectation that acylation of the 2-amino group reduces binding to DHFR. After treatment of this derivative with aminopeptidase M, the extent of inhibition correlated with the amount of MTX formed. MTX derivatives alone or in combination with the complementary peptidase were tested for cytotoxicity on murine L1210 cells in culture. The above-listed derivatives were considerably less cytotoxic than MTX, except for the L-leucyl derivative which showed considerable cytotoxicity. When the appropriate exogenous peptidase was included, the cytotoxicity of the activated prodrugs approached that of MTX. These results indicate that 2-L-leucyl-MTX is unsuitable as a prodrug since it is activated prematurely by serum enzymes. Although the L-valyl and L-isoleucyl derivatives do not hydrolyse to MTX in serum and are readily activated, they are not ideal prodrugs since they decompose under physiological conditions; the properties of the decomposition product will have a bearing on the ultimate suitability of these compounds. 2-L-Pyroglutamyl-MTX is the best candidate prodrug, showing stability and ready activation by the appropriate aminopeptidase.