Immunologically specific activation of a cephalosporin derivative of mitomycin C by monoclonal antibody beta-lactamase conjugates

Prodrugs in medicinal chemistry and enzyme prodrug therapies

Prodrugs are cunning derivatives of therapeutic agents designed to improve the pharmacokinetics profile of the drug. Within a prodrug, pharmacological activity of the drug is masked and is recovered within the human body upon bioconversion of the prodrug, a process that is typically mediated by enzymes. This concept is highly successful and a significant fraction of marketed therapeutic formulations is based on prodrugs. An advanced subset of prodrugs can be engineered such as to achieve site-specific bioconversion of the prodrug - to comprise the highly advantageous "enzyme prodrug therapy", EPT. Design of prodrugs for EPT is similar to the prodrugs in general medicinal use in that the pharmacological activity of the drug is masked, but differs significantly in that site-specific bioconversion is a prime consideration, and the enzymes typically used for EPT are non-mammalian and/or with low systemic abundance in the human body. This review focuses on the design of prodrugs for EPT in terms of the choice of an enzyme and the corresponding prodrug for bioconversion. We also discuss the recent success of "self immolative linkers" which significantly empower and diversify the prodrug design, and present methodologies for the design of prodrugs with extended blood residence time. The review aims to be of specific interest for medicinal chemists, biomedical engineers, and pharmaceutical scientists.

Targeted prodrug approaches for hormone refractory prostate cancer

Due to the propensity of relapse and resistance with prolonged androgen deprivation therapy (ADT), there is a growing interest in developing non-hormonal therapeutic approaches as alternative treatment modalities for hormone refractory prostate cancer (HRPC). Although the standard treatment for HRPC consists of a combination of ADT with taxanes and anthracyclines, the clinical use of chemotherapeutics is limited by systemic toxicity stemming from nondiscriminatory drug exposure to normal tissues. In order to improve the tumor selectivity of chemotherapeutics, various targeted prodrug approaches have been explored. Antibody-directed enzyme prodrug therapy (ADEPT) and gene-directed enzyme prodrug therapy (GDEPT) strategies leverage tumor-specific antigens and transcription factors for the specific delivery of cytotoxic anticancer agents using various prodrug-activating enzymes. In prostate cancer, overexpression of tumor-specific proteases such as prostate-specific antigen (PSA) and prostate-specific membrane antigen (PSMA) is being exploited for selective activation of anticancer prodrugs designed to be activated through proteolysis by these prostate cancer-specific enzymes. PSMA- and PSA-activated prodrugs typically comprise an engineered high-specificity protease peptide substrate coupled to a potent cytotoxic agent via a linker for rapid release of cytotoxic species in the vicinity of prostate cancer cells following proteolytic cleavage. Over the past two decades, various such prodrugs have been developed and they were effective at inhibiting prostate tumor growth in rodent models; several of these prodrug approaches have been advanced to clinical trials and may be developed into effective therapies for HRPC.

Syntheses and biological evaluation of new cephalosporin-oxazolidinone conjugates

Two cephalosporin-oxazolidinone conjugates were synthesized by incorporation of a carbamate linker at the 3'-position of the cephalosporin. These compounds show stability in aqueous media until specifically activated by a β-lactamase, and retain antibacterial activities profiles reflecting both the individual cephalosporin and oxazolidinone components.

Design and synthesis of a beta-lactamase activated 5-fluorouracil prodrug

An efficient synthesis of a 5-fluorouracil-cephalosporin prodrug is described for use against colorectal and other cancers in antibody and gene-directed therapies. The compound shows stability in aqueous media until specifically activated by beta-lactamase (betaL). The kinetic parameters of the 5-fluorouracil-cephalosporin conjugate were determined in the presence of Enterobacter cloacae P99 betaL (ECl betaL) revealing a K(m)=95.4 microM and V(max)=3.21 microMol min(-1) mg(-1). The data compare favorably to related systems that have been reported and enable testing of this prodrug against cancer cell lines in vitro and in vivo.

Enzyme-catalyzed activation of anticancer prodrugs

The rationale fo the development of prodrugs relies upon delivery of higher concentrations of a drug to target cells compared to administration of the drug itself. In the last decades, numerous prodrugs that are enzymatically activated into anti-cancer agents have been developed. This review describes the most important enzymes involved in prodrug activation notably with respect to tissue distribution, up-regulation in tumor cells and turnover rates. The following endogenous enzymes are discussed: aldehyde oxidase, amino acid oxidase, cytochrome P450 reductase, DT-diaphorase, cytochrome P450, tyrosinase, thymidylate synthase, thymidine phosphorylase, glutathione S-transferase, deoxycytidine kinase, carboxylesterase, alkaline phosphatase, beta-glucuronidase and cysteine conjugate beta-lyase. In relation to each of these enzymes, several prodrugs are discussed regarding organ- or tumor-selective activation of clinically relevant prodrugs of 5-fluorouracil, axazaphosphorines (cyclophosphamide, ifosfamide, and trofosfamide), paclitaxel, etoposide, anthracyclines (doxorubicin, daunorubicin, epirubicin), mercaptopurine, thioguanine, cisplatin, melphalan, and other important prodrugs such as menadione, mitomycin C, tirapazamine, 5-(aziridin-1-yl)-2,4-dinitrobenzamide, ganciclovir, irinotecan, dacarbazine, and amifostine. In addition to endogenous enzymes, a number of nonendogenous enzymes, used in antibody-, gene-, and virus-directed enzyme prodrug therapies, are described. It is concluded that the development of prodrugs has been relatively successful; however, all prodrugs lack a complete selectivity. Therefore, more work is needed to explore the differences between tumor and nontumor cells and to develop optimal substrates in terms of substrate affinity and enzyme turnover rates fo prodrug-activating enzymes resulting in more rapid and selective cleavage of the prodrug inside the tumor cells.

Synthesis of anticancer β-lactams: mechanism of action

Synthesis of the trans 1-N-chrysenyl and 1-N-phenanthrenyl 3-acetoxy-4-phenyl-2-azetidinones has been achieved. Microwave-assisted reaction has proved useful in the synthesis of these compounds. Cell growth inhibition study has indicated selective anticancer activity against two leukemia and colon carcinoma cell lines. A mechanistic correlation of their anticancer activity has been described. Striking G2 blockade that is clearly distinct in cell cycle analysis and demonstrated only in sensitive cell lines has been observed. They do not induce apoptosis in sensitive or resistant lines. They also do not inhibit topoisomerases. Ames test has shown they are nonmutagenic.

Improved yield and stability of L49-sFv-beta-lactamase, a single-chain antibody fusion protein for anticancer prodrug activation, by protein engineering

The L49 single-chain Fv fused to beta-lactamase (L49-sFv-bL) combined with the prodrug C-Mel is an effective anticancer agent against tumor cells expressing the p97 antigen. However, large-scale production of L49-sFv-bL from refolded E. coli inclusion bodies has been problematic due to inefficient refolding and instability of the fusion protein. Sequence analysis of the L49-sFv framework regions revealed three residues in the framework regions at positions L2, H82B, and H91, which are not conserved for their position, occurring in <1% of sequences in Fv sequence databases. One further unusual residue, found in <3% of variable sequences, was observed at position H39. Each unusual residue was mutated to a conserved residue for its position and tested for refolding yield from inclusion bodies following expression in E. coli. The three V(H) single mutants showed improvement in the yield of active protein and were combined to form double and triple mutants resulting in a 7-8-fold increased yield compared to the parental protein. In an attempt to further improve yield, the orientation of the triple mutant was reversed to create a bL-L49-sFv fusion protein resulting in a 3-fold increase in expressed inclusion body protein and producing a 20-fold increase in the yield of purified protein compared to the parental protein. The triple mutants in both orientations displayed increased stability in murine plasma and binding affinity was not affected by the introduced mutations. Both triple mutants also displayed potent in vitro cytotoxicity and in vivo antitumor activity against p97 expressing melanoma cells and tumor xenografts, respectively. These results show that a rational protein-engineering approach improved the yield, stability, and refolding characteristics of L49-sFv-bL while maintaining binding affinity and therapeutic efficacy.

Synthesis of beta-lactamase activated nitric oxide donors

In order to achieve site specific delivery of NO, we designed conjugates of cephalosporin with NO donors. NO donors such as cupferron and SIN-1 were evaluated as potential choices for conjugates. Cephalosporin conjugated with SIN-1 demonstrated promising beta-lactamase dependent NO releasing ability.

Selective activation of anticancer prodrugs by monoclonal antibody-enzyme conjugates

A great deal of interest has surrounded the activities of monoclonal antibodies (mAbs), and mAb-drug, toxin and radionuclide conjugates for the treatment of human cancers. In the last few years, a number of new mAb-based reagents have been clinically approved (Rituxan, Herceptin, and Panorex), and several others are now in advanced clinical trials. Successful therapeutic treatment of solid tumors with drug conjugates of such macromolecules must overcome the barriers to penetration within tumor masses, antigen heterogeneity, conjugated drug potency, and efficient drug release from the mAbs inside tumor cells. An alternative strategy for drug delivery involves a two-step approach to cancer therapy in which mAbs are used to localize enzymes to tumor cell surface antigens. Once the conjugate binds to the cancer cells and clears from the systemic circulation, antitumor prodrugs are administered that are catalytically converted to active drugs by the targeted enzyme. The drugs thus released are capable of penetrating within the tumor mass and eliminating both cells that have and have not bound the mAb-enzyme conjugate. Significant therapeutic effects have been obtained using a broad range of enzymes along with prodrugs that are derived from both approved anticancer drugs and highly potent experimental agents. This review focuses on the activities of several mAb-enzyme/prodrug combinations, with an emphasis on those that have provided mechanistic insight, clinical activity, novel protein constructs, and the potential for reduced immunogenicity.

Receptor-mediated and enzyme-dependent targeting of cytotoxic anticancer drugs

This review is a survey of various approaches to targeting cytotoxic anticancer drugs to tumors primarily through biomolecules expressed by cancer cells or associated vasculature and stroma. These include monoclonal antibody immunoconjugates; enzyme prodrug therapies, such as antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and bacterial-directed enzyme prodrug therapy; and metabolism-based therapies that seek to exploit increased tumor expression of, e.g., proteases, low-density lipoprotein receptors, hormones, and adhesion molecules. Following a discussion of factors that positively and negatively affect drug delivery to solid tumors, we concentrate on a mechanistic understanding of selective drug release or generation at the tumor site.

Development and activities of a new melphalan prodrug designed for tumor-selective activation

The synthesis of C-Mel, a cephalosporin carbamate derivative of the clinically used alkylating agent melphalan, is described. C-Mel was designed as an anticancer nitrogen mustard prodrug that releases melphalan upon tumor-specific activation by targeted beta-lactamase (bL). The Km and kcat values for bL hydrolysis of C-Mel were 218 microM and 980 s(-1), respectively. In vitro cytotoxicity assays with 3677 human melanoma cells demonstrated that C-Mel was 40-fold less toxic than melphalan and was activated in an immunologically specific manner by L49-sFv-bL, a recombinant fusion protein that binds to the melanotransferrin antigen on melanomas and on some carcinomas. L49-sFv-bL in combination with C-Mel led to regressions and cures of established subcutaneous 3677 tumors in nude mice. The effects were significantly greater than those of melphalan, which did not result in any long-term regressions in this tumor model. The therapeutic effects were comparable to those obtained in mice treated with the previously described L49-sFv-bL/7-(4-carboxybutanamido)-cephalosporin mustard (CCM) combination. However, C-Mel may be more attractive than CCM for clinical development since the released drug is clinically approved.