Nitroreductase: A Prodrug‐activating Enzyme for Cancer Gene Therapy

Abstract

1. The prodrug CB1954 (5-(aziridin-1-yl)-2,4-dinitrobenzamide) is activated by Escherichia coli nitroreductase (NTR) to a potent DNA-crosslinking agent. 2. Virus-mediated expression of NTR in tumour cells sensitizes them to CB1954 in vitro and in vivo, providing the basis for a strategy of cancer gene therapy. 3. A phase I trial of CB1954 in cancer patients has been completed, documenting the pharmacokinetics and establishing an acceptable dose. Subsequent trials of the replication-defective adenovirus CTL102 in patients with resectable tumours have documented expression of NTR in injected colorectal liver metastases, hepatocellular carcinoma, head and neck cancer and prostate cancer. Trials combining CTL102 and CB1954 are underway. 4. An oncolytic (replication-competent) adenovirus vector allowed increased expression of NTR in vitro and in a mouse tumour model, resulting in a greater reduction in tumour growth when combined with CB1954 treatment. 5. Alternative prodrugs may eventually prove superior to CB1954; a nitroaryl phosphoramide mustard prodrug activated by NTR shows a greater therapeutic index than CB1954 in a human ovarian carcinoma. 6. The crystal structure of NTR provided the basis for site-directed mutagenesis, which has identified a number of mutants with improved kinetics of CB1954 activation. These can provide improved cell sensitization to CB1954. Combinations of these are being tested. 7. The basis for a positive selection for improved NTR variants has been demonstrated.

Noninvasive optical imaging of nitroreductase gene-directed enzyme prodrug therapy system in living animals

“…24 More recently, a replication-defective adenoviral vector CTL102 encoding gene has been used on patients with primary or secondary liver cancer 25 and prostate cancer. 8,26 However, there are many challenges yet to be overcome for it to be considered a robust therapy. For any GDEPT system, there is a need for methods to monitor sufficient incorporation of the transgenes into tumor cells, gauge adequate level of gene expression and predict the duration of transgene expression in cancer Noninvasive optical imaging of NTR GDEPT system S Bhaumik et al cells.…”

The nitroreductase prodrug SN 28343 enhances the potency of systemically administered armed oncolytic adenovirus ONYX-411NTR

“…[52][53][54][55] Phase I trials of CB 1954, alone 56 and in conjunction with a non-replicating NTR-armed adenovirus have been conducted. 57,58 However, CB 1954 has poor formulation characteristics 56 that have not been overcome by analog development 59,60 and a limited bystander effect in 3D cell culture models and xenografts. 46,61,62 Of critical importance, CB 1954 provides disappointing plasma concentrations in humans (5.8 mmol h l À1 ) 56 relative to mice (320 mmol h l À1 ) 63 because of dose-limiting hepatotoxicity and diarrhea.…”

E. coli NfsA: an alternative nitroreductase for prodrug activation gene therapy in combination with CB1954

“…Anlezark et al (1992) reported that the nitroreductase encoded by the nfsB gene of Escherichia coli could activate CB1954, leading to the initial adoption of this enzyme for use with CB1954 in GDEPT Bridgewater et al, 1995;Grove et al, 1999;Searle et al, 2004). The expression of NfsB in cancer cells using replicationdefective retrovirus or adenovirus vectors was shown to confer greatly increased sensitivity to CB1954, and anti-tumour activity was shown in vivo (McNeish et al, 1998;Djeha et al, 2000Djeha et al, , 2001Weedon et al, 2000).…”