Purpose Tirapazamine (TPZ) has attractive features for targeting hypoxic cells in tumors but limited clinical activity, in part because of poor extravascular penetration. Here we identify improved TPZ analogs by using a spatially resolved pharmacokinetic/pharmacodynamic (SR-PKPD) model that considers tissue penetration explicitly during lead optimization. Experimental design The SR-PKPD model was used to guide progression of 281 TPZ analogs through a hierarchical screen. For compounds exceeding hypoxic selectivity thresholds in single cell cultures, SR-PKPD model parameters (kinetics of bioreductive metabolism, clonogenic cell killing potency, diffusion coefficients in multicellular layers, plasma pharmacokinetics at well tolerated doses in mice) were measured to prioritize testing in xenograft models in combination with radiation. Results SR-PKPD-guided lead optimization identified SN29751 and SN30000 as the most promising hypoxic cytotoxins from two different structural subseries. Both were reduced to the corresponding 1-oxide selectively under hypoxia by HT29 cells, with an oxygen dependence quantitatively similar to that of TPZ. SN30000, in particular, showed higher hypoxic potency and selectivity than TPZ in tumor cell cultures and faster diffusion through HT29 and SiHa multicellular layers. Both compounds also provided superior plasma PK in mice and rats at equivalent toxicity. In agreement with SR-PKPD predictions, both were more active than TPZ with single dose or fractionated radiation against multiple human tumor xenografts. Conclusions SN30000 and SN29751 are improved TPZ analogs with potential for targeting tumor hypoxia in humans, and illustrate the utility of novel SR-PKPD modeling approaches for lead optimization during anticancer drug development.

Hypoxia represents an important therapeutic target in tumors because of the resistance of hypoxic cells to radiotherapy and chemotherapy and because it is more severe in many tumors than in normal tissues. Here, we describe a class of prodrugs, nitro-chloromethylindolines, which undergo hypoxia-selective activation by endogenous nitroreductases in tumor cells to form the corresponding amino compounds. The latter are chemically related to the cyclopropylindoline antitumor antibiotics and they share the same properties of sequence-selective DNA minor groove alkylation and high cytotoxic potency. Of three alkylating subunits investigated, the chloromethylbenzindoline (CBI) structure provided the most favorable prodrug properties: aerobic cytotoxic potency of the amines was approximately 90-to 3,000-fold higher than the corresponding nitro compounds, and the nitro compounds showed air/anoxia potency differentials of up to 300-fold. Selective alkylation of adenine N3 in calf thymus DNA by an amino-CBI was shown by characterization of the thermal depurination product; the same adduct was shown in hypoxic RIF-1 cells exposed to the corresponding nitro-CBI prodrug under hypoxic (but not oxic) conditions. The amino metabolite generated from a nitro-CBI by cells expressing Escherichia coli nfsB nitroreductase in multicellular layer cultures was shown to elicit bystander killing of surrounding cells. Nitro-CBI prodrugs were >500-fold less toxic to mice than amino-CBIs by i.p. administration and provided selective killing of hypoxic cells in RIF-1 tumors (although only at maximally tolerated doses). Nitro-CBIs are novel lead hypoxia-activated prodrugs that represent the first examples of hypoxiaselective generation of potent DNA minor groove alkylating agents.