Title: Cancer Research UK Centre for Cancer Therapeutics - Gene and oncogene targeting team
Funder: Cancer Research UK

Abstract: Our principal research interest is in the area of enzyme prodrug therapy for cancer. Gene-directed enzyme prodrug therapy (GDEPT) is a two step therapy for cancer treatment. In the first step, a gene for the prodrug-activating enzyme is delivered to tumour cells. A non-toxic prodrug is administered in the second step. Expression of the foreign enzyme in the tumour results in conversion of the prodrug into a cytotoxic drug able to kill the tumour cells. We have used two novel enzymes for GDEPT with the aim of delivering drugs to tumours that would not require cellular activation and would be active against quiescent as well as proliferating cells: (1) nitroreductase (NR) to activate the prodrug CB1954 and (2) carboxypeptidase G2 (CPG2), in conjunction with the prodrug CJS8, the bifunctional alkylating agent, 4-[(2-chloroethyl)(2-mesyloxyethyl)amino]benzoyl-L-glutamic acid (also known as CMDA). CPG2 cleaves CJS8 to release the cytotoxic drug, 4-[(2-chloroethyl)(2-mesyloxyethyl)amino]benzoic acid, directly, with the advantage that no further enzymatic processing is required for drug activation. We have been the first group to show that surface tethering of enzyme, in collaboration with Dr R Marais (CBL) is a beneficial strategy in GDEPT. We have designed a wide range of novel prodrugs that have been synthesised for activation by CPG2. Initially we concentrated on mustard prodrugs with a direct activator functionality. More recently we have concentrated on self immolative prodrugs with an indirect activator group and other non-mustard prodrugs. This has greatly expanded the scope of drugs that can be released. The physical, kinetic and biological properties of these prodrugs activated by CPG2 have been assessed in cells engineered to express intracellular and surface tethered CPG2. Several prodrugs have been identified that are more potent and produced a larger cytotoxic differential than the lead compound CJS8. These have shown exciting results that are superior to CJS8. For example, we have demonstrated complete cures in xenograft models that have been engineered to express CPG2. We are currently working on incorporating the CPG2 into novel vectors that are able to deliver CPG2 to tumours following intratumoural or systemic administration. In addition, we have retained involvement with the current Antibody-Directed Enzyme Prodrug Therapy (ADEPT) clinical trials. We have performed pharmacokinetic measurements on the prodrugs from patient samples. Two clinical trials have been performed on the prodrug CJS8. In the second Phase I/II ADEPT clinical trial, 10 patients with colorectal cancer were treated. The anti-CEA antibody (Ab), A5B7, was fragmented to F(ab')2 and chemically conjugated to CPG2, administered to patients and followed by a galactosylated clearing antibody. Pharmacokinetic parameters, enzyme measurements and clinical observations confirmed that ADEPT gives selective delivery to tumour tissue, with one patient having a partial response and 6 patients having stable disease for a median of 4 months after previous tumour progression. A third clinical trial from 1997-1999, at the Royal Free Hospital, employed the same Ab-enzyme conjugate in 28 patients with escalating doses of a more potent prodrug, CJS149. No clearing antibody was used as the shorter half-life of the prodrug and drug was thought to prevent systemic toxicity. One patient showed a clinical response. A fourth trial is ongoing.


Research activity codes: 6.2 CELLULAR AND GENE THERAPIES;5.2 CELLULAR AND GENE THERAPIES
Health categories: CANCER AND NEOPLASMS