Bioreductively Activatable Prodrug Conjugates of Combretastatin A‑1 and Combretastatin A‑4 as Anticancer Agents Targeted toward Tumor-Associated Hypoxia

The natural products combretastatin
A-1 (CA1) and combretastatin
A-4 (CA4) function as potent inhibitors of tubulin polymerization
and as selective vascular disrupting agents (VDAs) in tumors. Bioreductively
activatable prodrug conjugates (BAPCs) can enhance selectivity by
serving as substrates for reductase enzymes specifically in hypoxic
regions of tumors. A series of CA1-BAPCs incorporating nor-methyl, mono-methyl, and gem-dimethyl
nitrothiophene triggers were synthesized together with corresponding
CA4-BAPCs, previously reported by Davis (Mol. Cancer Ther. 2006, 5 (11), 2886), for comparison. The CA4-gem-dimethylnitrothiophene BAPC 45 proved exemplary
in comparison to its nor-methyl 43 and mono-methyl 44 congeners. It was stable in
phosphate buffer (pH 7.4, 24 h), was cleaved (25%, 90 min) by NADPH-cytochrome
P450 oxidoreductase (POR), was inactive (desirable prodrug attribute)
as an inhibitor of tubulin polymerization (IC50 > 20
μM),
and demonstrated hypoxia-selective activation in the A549 cell line
[hypoxia cytotoxicity ratio (HCR) = 41.5]. The related CA1-gem-dimethylnitrothiophene BAPC 41 was also
promising (HCR = 12.5) with complete cleavage (90 min) upon treatment
with POR. In a preliminary in vivo dynamic bioluminescence imaging
study, BAPC 45 (180 mg/kg, ip) induced a decrease (within
4 h) in light emission in a 4T1 syngeneic mouse breast tumor model,
implying activation and vascular disruption.