Caroline Joy Springer

A cytotoxic agent can be generated selectively at cancer sites.

Attempts to improve the selectivity of anti-cancer agents by conjugating them to antibodies directed at tumour associated antigens have demonstrated tumour localisation but only limited therapeutic success. We report here the advantage of a 2-stage approach in which the first component combines the selective delivery of antibody with a capability to generate a cytotoxic agent from a second subsequently administered component. A bacterial enzyme, carboxypeptidase G2 (CPG2) was conjugated with F(ab)2 fragment of a monoclonal antibody directed at beta subunit of human chorionic gonadotrophin (beta-hCG) and injected into nude mice bearing hCG producing CC3 xenografts of human choriocarcinoma. Time was allowed for the conjugate to localise at tumour sites and clear from blood before injecting para-N-bis (2-chloroethyl) aminobenzoylglutamic acid. Cleavage of the glutamic acid moiety from this molecule by CPG2 released a benzoic acid mustard. Growth of the tumour which is resistant to conventional chemotherapy was markedly depressed by a single course of treatment. This demonstrates for the first time the potential of an antibody directed enzyme to activate an alkylating agent and to eradicate an established human cancer xenograft.

Ablation of Human Choriocarcinoma Xenografts in Nude Mice by Antibody-directed Enzyme Prodrug Therapy (ADEPT) with Three Novel Compounds

Three novel prodrugs have been designed for use as anticancer agents. Each is a bifunctional alkylating agent which has been protected to form a relatively inactive prodrug. They are designed to be activated to their corresponding alkylating agents at a tumour site by prior administration of an antitumour antibody conjugated to the bacterial enzyme carboxypeptidase G2 (CPG2) in a two-phase system called antibody-directed enzyme prodrug therapy (ADEPT). The Km and Vmax values for three different antibody-CPG2 conjugates were determined in relation to each prodrug. The Km values ranged from 4.5-12 mumol/l and the Vmax from 0.5-1.6 mumol/U/min. Athymic Nu/Nu mice with palpable transplanted human choriocarcinoma xenografts, which are resistant to conventional chemotherapy, were treated with anti-human chorionic gonadotropin antibodies conjugated to CPG2. This was followed by each of the three novel prodrugs. Significant increase in survival was obtained in three of the regimens tested using only one course of treatment. This demonstrates the potential of a tumour-localised bacterial enzyme to activate protected alkylating agents in order to eradicate an established human xenograft.

The cytotoxicity, DNA crosslinking ability and DNA sequence selectivity of the aniline mustards melphalan, chlorambucil and 4-[bis(2-chloroethyl)amino] benzoic acid

Three aniline derivatives melphalan (L-PAM), chlorambucil (CHL) and 4-[bis(2-chloroethyl)amino] benzoic acid (BAM) have been compared on the basis of their in vitro cytotoxicities, DNA interstrand crosslinking ability and DNA sequence selectivity. Cytotoxicity was assessed in the human colonic adenocarcinoma LS174T and leukaemic K562 cell lines using the sulpho-rhodamine B and tetrazolium dye reduction assays. The order of cytotoxicities was L-PAM greater than CHL greater than BAM in both cell lines with K562 being less sensitive than LS174T. This was different from the order CHL greater than L-PAM greater than BAM which would be predicted from simple chemical reactivity or rate of hydrolysis, parameters which have been used previously as indicators of biological potency for aromatic nitrogen mustards. DNA interstrand crosslinking in cells as determined by alkaline elution showed a correlation with IC50 values. The ranking order of activity was further predicted by the ability of the agents to produce interstrand crosslinks in isolated DNA. The extent of guanine N-7 alkylation, assessed using a modified DNA sequencing technique, mirrored cytotoxicity and crosslinking ability, but at equivalent levels of alkylation there was no significant difference in DNA sequence selectivity. These data demonstrates that simple chemical reactivity or hydrolysis rate is not a good indicator of DNA reactivity or cytotoxicity for a number of aniline mustards, whereas DNA interstrand crosslinking ability either measured directly in cells or in isolated DNA, gives a good indication of biological activity.

Galactosylated antibodies and antibody‐enzyme conjugates in antibody‐directed enzyme prodrug therapy

Antibody directed enzyme prodrug therapy (ADEPT) has been studied as a two‐ and three‐phase system in which an antibody to a tumor‐associated antigen has been used to deliver an enzyme to tumor sites where it can convert a relatively nontoxic prodrug to a cytotoxic agent. In such a system, it is necessary to allow the enzyme activity to clear from the blood before prodrug injection to avoid toxicity caused by prodrug activation in plasma. To accelerate plasma clearance of enzyme activity, two approaches have been studied. The studies have been performed with a monoclonal anticarcinoembryonic‐antigen antibody fragment A5B7‐F(ab′)2 conjugated to a bacterial enzyme, carboxypeptidase G2 (CPG2), in LS174T xenografted mice. In the first approach, a monoclonal antibody (SB43), directed at CPG2, was used, which inactivates CPG2 in vitro and in vivo. SB43 was galactosylated so that it had sufficient time to form a complex with plasma CPG2, resulting in the inactivation and clearance of the complex from plasma via the carbohydrate‐specific receptors in the liver. Injection of SB43gal 19 hours after administration of the radiolabeled conjugate reduced the percentage of injected dose per gram in blood without affecting levels in the tumor.

Anti-tumour effects of an antibody-carboxypeptidase G2 conjugate in combination with phenol mustard prodrugs.

ADEPT is an antibody-based targeting strategy for the treatment of cancer. We have developed two new prodrugs, 4-[N,N-bis(2-chloroethyl)amino]-phenoxycarbonyl-L- glutamic acid (PGP) and (S)-2-[N-[4-[N,N-bis(2-chloroethyl)amino]- phenoxycarbonyl]amino]-4-(5-tetrazoyl)butyric acid (PTP), which are cleaved by the bacterial enzyme CPG2 to release the 4-[N,N-bis(2-chloroethyl)amino] phenol drug. In vitro, both prodrugs are approximately 100- to 200-fold less potent than the parent drug (1 h IC50 = 1.4 microM) in LoVo colorectal tumour cells. These prodrugs have been evaluated for utility in ADEPT when used in combination with a conjugate of CPG2 and the F(ab)2 fragment of the anti-CEA monoclonal antibody, A5B7. The conjugate was shown to localise specifically to established LoVo tumour xenografts growing in nude mice and optimal tumour-normal tissue ratios were achieved after 72 h. Administration of either prodrug, at doses which cause 6-8% body weight loss, 72 h after administration of the A5B7-CPG2 conjugate to the LoVo tumour-bearing mice resulted in tumour regressions and growth delays of 14-28 days. The PTP prodrug in combination with a high dose of conjugate (10 mg kg-1) gave the best anti-tumour activity despite being a 10-fold worse substrate for CPG2 than PGP. Prodrug alone, active drug alone or prodrug in combination with a non-specific conjugate had minimal anti-tumour activity in this tumour model.

Antibody-directed enzyme prodrug therapy (ADEPT): A three-phase study in ovarian tumor xenografts

Antibody-directed enzyme prodrug therapy (ADEPT) has been studied in a human ovarian carcinoma xenograft grown subcutaneously in nude mice. Radioimmunoassay of supernatants obtained from tumor homogenates showed these to contain carcinoembryonic antigen (CEA). Biodistribution studies with 125I-labeled monoclonal anti-CEA antibody, A5B7, and its F(ab)2 fragment showed localization in these xenografts. The AB57-F(ab)2 fragment conjugated to a bacterial enzyme, carboxypeptidase G2 (CPG2), and, radiolabeled with 125iodine, also localized in the xenografts. The radiolabeled conjugate cleared from blood faster than the antibody alone. The percentage of injected dose per gram in tumor at 24 h postinjection was about fivefold lower than antibody alone. Tumor-to-blood ratio at 72 h after injection of the radiolabeled conjugate was 7 and the tumor-to-normal tissue ratios at this time point ranged from 20 (liver) to 75 (colon). A three-phase ADEPT antitumor study was carried out in which A5B7-F(ab)2-CPG2 was allowed to localize and was followed by accelerated inactivation/clearance of blood CPG2 by a galactosylated anti-CPG2 antibody (SB43gal). A benzoic acid mustard-derived prodrug was injected 24 h after the conjugate, which led to growth delay in this tumor compared to the control untreated group. Further antitumor studies in this model are in progress.Antibody-directed enzyme prodrug therapy (ADEPT) has been studied in a human ovarian carcinoma xenograft grown subcutaneously in nude mice. Radioimmunoassay of supernatants obtained from tumor homogenates showed these to contain carcinoembryonic antigen (CEA). Biodistribution studies with125I-labeled monoclonal anti-CEA antibody, A5B7, and its F(ab′)2 fragment showed localization in these xenografts. The AB57-F(ab′)2 fragment conjugated to a bacterial enzyme, carboxypeptidase G2 (CPG2), and, radiolabeled with125iodine, also localized in the xenografts. The radiolabeled conjugate cleared from blood faster than the antibody alone. The percentage of injected dose per gram in tumor at 24 h postinjection was about fivefold lower than antibody alone. Tumor-to-blood ratio at 72 h after injection of the radiolabeled conjugate was 7 and the tumor-to-normal tissue ratios at this time point ranged from 20 (liver) to 75 (colon).A three-phase ADEPT antitumor study was carried out in which A5B7-F(ab′)2-CPG2 was allowed to localize and was followed by accelerated inactivation/clearance of blood CPG2 by a galactosylated anti-CPG2 antibody (SB43gal). A benzoic acid mustard-derived prodrug was injected 24 h after the conjugate, which led to growth delay in this tumor compared to the control untreated group. Further antitumor studies in this model are in progress.

Synthesis of an N-mustard prodrug

Abstract We describe the synthesis of the novel N-mustard prodrug 4-[(2-chloroethyl)(2-mesyloxyethyl)amino]benzoyl-L-glutamic acid, which is designed for activation by tumour localising antibody-carboxypeptidase conjugates.