Targeting: the ADEPT story so far.

Antibody-directed enzyme prodrug therapy aims to restrict the action of a cytotoxic drug to cancer sites. An enzyme that has no human analogue is delivered to cancer sites by attachment to an antibody directed at a tumour associated antigen. In a second step an antibody or other agent inactivates and clears enzyme from blood. The third step is administration of a low toxicity prodrug that is a substrate for the enzyme thus generating a potent cytotoxic agent at cancer sites. Encouraging results were obtained with this system in small scale clinical trials using unrefined agents. During the past 10 years attempts have been made to reduce the system to two components. Although these have met with some success it is now accepted that future progress requires all three components.

Antibody-directed enzyme prodrug therapy: efficacy and mechanism of action in colorectal carcinoma.

In antibody-directed enzyme prodrug therapy, an enzyme conjugated to an antitumor antibody is given i.v. and localizes in the tumor. A prodrug is then given, which is converted to a cytotoxic drug selectively in the tumor. Ten patients with colorectal carcinoma expressing carcinoembryonic antigen received antibody-directed enzyme prodrug therapy with A5B7 F(ab')2 antibody to carcinoembryonic antigen conjugated to carboxypeptidase G2 (CPG2). A galactosylated antibody directed against the active site of CPG2 (SB43-gal) was given to clear and inactivate circulating enzyme. A benzoic acid mustard-glutamate prodrug was given when plasma enzyme levels had fallen to a predetermined safe level, and this was converted by CPG2 in the tumor into a cytotoxic form. Enzyme levels derived from quantitative gamma camera imaging and from direct measurements in plasma and tumor biopsies showed that the median tumor:plasma ratio of enzyme exceeded 10000:1 at the time of prodrug administration. Enzyme concentrations in the tumor (median, 0.47 units g(-1)) were sufficient to generate cytotoxic levels of active drug. The concentration of prodrug needed for optimal conversion (Km) of 3 microM was achieved. Prodrug conversion to drug was shown by finding detectable levels of drug in plasma. There was evidence of tumor response; one patient had a partial response, and six patients had stable disease for a median of 4 months after previous tumor progression (one of these six had a tumor marker response). Manageable neutropenia and thrombocytopenia occurred. Conditions for effective antitumor therapy were met, and there was evidence of tumor response in colorectal cancer.

Developments with targeted enzymes in cancer therapy.

Cancer therapy based on the delivery of enzymes to tumour sites has advanced in several directions since antibody-directed enzyme/prodrug therapy was first described. It has been shown that methoxypolyethylene glycol (MPEG) can be used to deliver enzyme to a variety of solid tumours. MPEG-enzyme conjugates show reduced immunogenicity and may allow repeated treatment with enzymes of bacterial origin. Enzyme delivery to tumours by polymers can be used to convert a low toxicity prodrug to a potent cytotoxic agent. An example of such a prodrug is CB1954, which can be activated by a human enzyme in the presence of a cosubstrate. Tumour-located enzymes can also be used in conjunction with a combination of antimetabolites and rescue agents. The rescue agent protects normal tissue but is degraded at cancer sites by the enzyme, thus deprotecting the tumour and allowing prolonged antimetabolite action.

Targeting enzymes to cancers--new developments.

Two methods of using tumour located enzymes have been described. These are antibody directed enzyme prodrug therapy (ADEPT) and macromolecule directed enzyme prodrug therapy (MDEPT), where the tumour located enzyme converts a non-toxic prodrug into a cytotoxic drug at tumour sites. The alternative use of tumour located enzymes is to inactivate rescue agents that protect cells from antimetabolite action, and is described as 'Antimetabolite with inactivation of rescue agent at cancer sites' (AMIRACS). The leakiness of tumour blood vessels and poor lymphatic drainage allows enzymes to be targeted to many cancers by attachment to polymeric macromolecules (MDEPT), as well as to antibodies and antibody fragments (ADEPT). To avoid systemic toxicity, enzyme activity in blood and normal tissues must be very low before giving a prodrug or rescue agent. Antibodies directed against the enzyme component of macromolecular conjugates have proved to be very efficient at clearing normal tissues. Human enzymes which are over expressed by cancer cells can be exploited particularly if they require co-factors or co-substrates, either in situ or targeted to extracellular sites. Bacterial enzymes have advantages in specificity but require some form of immunological control in view of their immunogenicity. Prodrugs which generate drugs with very short half lives are desirable, and have been developed, including one which has a differential toxicity between prodrug and the active drug of 1000 to 10,000 fold. The range of antimetabolites available for AMIRACS was initially restricted to inhibitors of dihydrofolate reductase but has been greatly extended by the introduction of inhibitors of other enzymes. The limitations of these systems are discussed.

The management of high-risk gestational trophoblastic tumours (GTT).

Between 1979 and 1995 we have treated 272 consecutive women with high-risk (GTT including 121 previously treated patients who were treated with the weekly EMA/CO (etoposide, methotrexate, actinomycin D alternating with cyclophosphamide and vincristine). The median follow-up is 4.5 years (range 1-16 years). The cumulative 5 year survival is 86.2% (95% confidence interval 81.9-90.5%). No deaths from GTT occurred later than 2 years after starting EMA/CO. In a multivariate analysis, adverse prognostic factors were the presence of liver metastases (p < 0.0001), interval from antecedent pregnancy > 24 months (p < 0.0001), brain metastases (p=0.0008) and term delivery of antecedent pregnancy (p=0.045). There were 11 (4%) early deaths while 213 (78%) achieved complete remission. 47 (17%) developed drug resistance to EMA/CO of whom 33 (70%) were salvaged by further cisplatinum based chemotherapy and surgery. 2 women developed acute myeloid leukaemia after treatment with EMA/CO. 56% of women who have been in remission for at least 2 years and had fertility conserving surgery have achieved pregnancy since completing EMA/CO and there have been 112 live births including 3 babies with congenital abnormalities. EMA/CO is an effective, easy to administer and well tolerated regimen for treating patient with high-risk GTT. More than half of these women will retain their fertility. However, there is a small but significant increase in second malignancies.

Chemotherapy for gestational trophoblastic tumours hastens menopause by 3 years.

Chemotherapy may induce acute ovarian failure, but in women who retain gonadal function throughout chemotherapy, the late effects upon ovarian function are unknown. A retrospective controlled survey was performed to ascertain whether chemotherapy for gestational trophoblastic tumours (GTT) results in premature menopause. Questionnaires were sent to 1,489 women diagnosed between 1971 and 1990 with GTT, including 1089 who had received chemotherapy and 400 who had not received chemotherapy (controls). Responses were obtained from 972 chemotherapy-treated patients and 327 controls. 124 women were not evaluable for menopause date as they had undergone hysterectomy as part of the treatment for GTT or had developed permanent amenorrhoea during chemotherapy. Overall, 172 women reported that they were postmenopausal, including 157 women who had received chemotherapy. The median age at menopause for the evaluable population was 50 years (range 25-56 years). The age at menopause was significantly earlier in the treated arm (median 50, range 25-56 years) than in the controls (median 53, range 40-57 years) (logrank test chi 2 = 12.6, P = 0.0004). Menopause occurred significantly earlier in women treated with combination chemotherapy (median 49, range 25-56 years) compared with single agent methotrexate (median 51, range 25-56 years) (logrank test chi 2 = 8.3, P = 0.004). However, the age at completion of chemotherapy in the treated arm did not influence the age of menopause (proportional Hazards chi 2 = 1.99, P = 0.16). Chemotherapy for GTT induces menopause 3 years earlier than it occurs in women with GTT who do not receive chemotherapy. Although the difference is statistically significant, the magnitude is modest and most women can be reassured that neither fertility nor postmenopausal osteoporosis will be greatly affected.

Management of resistant gestational trophoblastic tumors.

OBJECTIVE: To analyze the causes of therapeutic success and failure in the management of patients with high-risk gestational trophoblastic tumors (GTTs). STUDY DESIGN: Analysis of 272 consecutive high-risk patients treated at the trophoblastic disease center at the Charing Cross Hospital between 1979 and 1995. RESULTS: EMA (etoposide, methotrexate, actinomycin D)/CO (cyclophosphamide, vincristine) chemotherapy is our treatment of choice for patients with high-risk GTT. In 272 consecutive patients the cumulative five-year survival was 86.2% (95% confidence interval, 81.9-90.5%). No deaths occurred from GTT more than two years after the start of treatment. In patients whose disease became resistant to EMA/CO or relapsed after receiving EMA/CO, the majority (70%) could be salvaged with further chemotherapy (usually with the EP (etoposide, cisplatin)/EMA chemotherapy with or without surgery. Multivariate analysis identified the following adverse prognostic factors: presence of liver metastases (P < .0001), prolonged interval from antecedent pregnancy (P < .0001), presence of brain metastases (P = .0008) and term delivery of antecedent pregnancy (P = .045). Intensive chemotherapy for treating high-risk GTT carries a small risk of inducing second malignancies, and two patients developed acute myeloid leukemia, 2 cervical malignancy and 1 gastric adenocarcinoma after receiving EMA/CO chemotherapy. CONCLUSION: EMA/CO is an effective and well-tolerated regimen for high-risk GTT. Salvage chemotherapy with EP/EMA is effective in the majority of patients whose disease is resistant to EMA/CO and should be combined with surgery when the dominant site of resistant disease is known. Major adverse prognostic variables have been identified, and patients with combinations of these factors should be considered for innovative therapeutic approaches from the outset.

The management of high-risk gestational trophoblastic tumours (GTT).

Between 1979 and 1995 we have treated 272 consecutive women with high-risk (GTT including 121 previously treated patients who were treated with the weekly EMA/CO (etoposide, methotrexate, actinomycin D alternating with cyclophosphamide and vincristine). The median follow-up is 4.5 years (range 1-16 years). The cumulative 5 year survival is 86.2% (95% confidence interval 81.9-90.5%). No deaths from GTT occurred later than 2 years after starting EMA/CO. In a multivariate analysis, adverse prognostic factors were the presence of liver metastases (p < 0.0001), interval from antecedent pregnancy > 24 months (p < 0.0001), brain metastases (p = 0.0008) and term delivery of antecedent pregnancy (p = 0.045). There were 11 (4%) early deaths while 213 (78%) achieved complete remission. 47 (17%) developed drug resistance to EMA/CO of whom 33 (70%) were salvaged by further cisplatinum based chemotherapy and sugery. 2 women developed acute myeloid leukaemia after treatment with EMA/CO. 56% of women who have been in remission for at least 2 years and had fertility conserving surgery have achieved pregnancy since completing EMA/CO and there have been 112 live births including 3 babies with congenital abnormalities. EMA/CO is an effective, easy to administer and well tolerated regimen for treating patient with high-risk GTT. More than half of these women will retain their fertility. However, there is a small but significant increase in second malignancies.

EMA/CO for high-risk gestational trophoblastic tumors: results from a cohort of 272 patients.

PURPOSE: To evaluate the results of etoposide, methotrexate, and dactinomycin alternating with cyclophosphamide and vincristine (EMA/CO) chemotherapy in women with high-risk gestational trophoblastic tumors (GTT) and to document the middle- and long-term toxicity of the regimen. PATIENTS AND METHODS: A total of 272 consecutive women with high-risk GTT, including 121 previously treated patients, were treated with weekly EMA/CO. The median follow-up duration is 4.5 years (range, 1 to 16). RESULTS: The cumulative 5-year survival rate is 86.2% (95% confidence interval, 81.9% to 90.5%). No deaths from GTT have occurred later than 2 years after the end [corrected] of EMA/CO. In a multivariate model, adverse prognostic factors were the presence of liver metastases (P < .0001), interval from antecedent pregnancy (P < .0001), brain metastases (P = .0008), and term delivery of antecedent pregnancy (P = .045). There were 11 (4%) early deaths, while 213 patients (78%) achieved a complete remission. Forty-seven (17%) developed drug resistance to EMA/CO, of whom 33 (70%) were salvaged by further cisplatin-based chemotherapy and surgery. Two women developed acute myeloid leukemia, two cervical malignancy, and one gastric adenocarcinoma after EMA/CO. More than half (56%) of the women who had fertility-conserving surgery and who have been in remission at least 2 years have become pregnant since the completion of EMA/CO, with 112 live births, including three infants with congenital abnormalities. CONCLUSION: EMA/CO is an effective and well-tolerated regimen for high-risk GTT. More than half of the women will retain their fertility; however, there is a small but significant risk of second malignancy.

Gestational trophoblastic disease with liver metastases: the Charing Cross experience.

OBJECTIVE: To define management options for women presenting with gestational trophoblastic disease (GTD) which had already metastasised to the liver. DESIGN: Retrospective analysis of case records between 1958 and 1994. SETTING: A national referral centre for trophoblastic disease. RESULTS: The database containing 1676 treated patients was reviewed and 46 patients with hepatic metastases were identified (2.7%). The median age was 32 years (range 19-52 years). The antecedent pregnancy to the GTD was normal in 65% (30/46), and the time interval between the antecedent pregnancy and presentation was longer than one year in 50% (22/44). Lung metastases were present in 43 patients (93%) and brain deposits in 15 patients (33%). Forty-five patients (98%) were high risk by WHO criteria. The five-year overall survival was 27%. The five-year survival of the subgroup of patients having both hepatic and cerebral metastases was 10%. There was no significant survival difference between the different chemotherapy regimens used in the study period (pre-1979 CHAMOCA: methotrexate, actinomycin D, cyclophosphamide, doxorubicin, melphalan, hydroxyurea and vincristine; 1979 onwards EMA/CO-EP: etoposide, methotrexate, adriamycin-D/ cyclophosphamide, vincristine-etoposide and cis-platinum). Multivariate analysis revealed that a prognostic score > 12 was significant (Hazard ratio 5.4, 95% CI 0.7-41.9; P = 0.04). CONCLUSIONS: The outcome for women presenting with hepatic metastases from GTD is poor with an even worse prognosis if cerebral metastases are also present. Alternative therapeutic measures, such as high dose therapy or new drugs, should be explored in these women.

Combination but not single-agent methotrexate chemotherapy for gestational trophoblastic tumors increases the incidence of second tumors.

PURPOSE: No increase in second tumor incidence was found in a previous analysis of women treated with chemotherapy for gestational trophoblastic tumors (GTT). More patient years at risk enabled a further analysis of the risk of second tumors to be performed in the 1,377 women treated in this until up to 1990. PATIENTS AND METHODS: Health questionnaires were returned on 93.3% of patients who successfully completed chemotherapy and were living in the United Kingdom. The remainder were flagged for death or developing further cancers by the Office of Population Census and Surveys and by the Thames Cancer Registry. Incidence density analysis was performed based on 15,279 person-years of observation available. Standardized incidence ratio (SIR) was used to estimate the relative risk (RR) of second tumors associated with the treatment. To calculate the expected number, the actual incidence rates observed by the Thames Cancer Registry during the same calendar period of observation were used. RESULTS: An overall 50% excess of risk (RR = 1.5; 95% confidence interval [CI], 1.1 to 2.1; P < .011) was observed: there were 37 second tumors, when 24.5 were expected. For specific second tumors, the risk was significantly increased for myeloid leukemia (RR = 16.6; 95% CI, 5.4 to 38.9), colon (RR = 4.6; 95% CI, 1.5 to 10.7), and breast cancer when the survival exceeded 25 years (RR = 5.8; 95% CI, 1.2 to 16.9). The risk was not significantly increased among the 554 women receiving single-agent therapy (RR = 1.3; 95% CI, 0.6 to 2.1). Leukemias only developed in patients receiving etoposide plus other cytotoxic drugs. CONCLUSION: This study suggests that there is a slight increased risk of second tumors after sequential or combination chemotherapy for GTT. This has become apparent since the introduction of etoposide and longer follow-up.

Placental site trophoblastic tumor: molecular analysis and clinical experience.

Placental site trophoblastic tumor is a very rare variant of gestational trophoblastic disease which differs histologically and immunocytochemically from gestational choriocarcinoma. The English language literature includes only 74 reported cases. Seventeen patients have been managed at Charing Cross Hospital with this diagnosis. The median follow-up is 4.6 years, and the 5-year overall survival is 80% (95% confidence interval, 55-93%). Multivariate regression analysis identified an interval of >2 years since the preceding pregnancy as an independent adverse prognostic factor. Genotypic analysis by PCR allelotyping has confirmed the gestational origin of all 11 tumors successfully studied. More detailed molecular analysis has identified the causative pregnancy for eight tumors. Five were diploid biparental tumors following term pregnancies, and three were androgenetic tumors following monospermic complete hydatidiform moles.

A clinical, histopathological and flow cytometric study of 149 complete moles, 146 partial moles and 107 non-molar hydropic abortions.

We have compared the clinical and histological features of 149 complete moles with 146 triploid partial moles and 107 diploid non-molar hydropic abortions initially registered as moles for human chorionic gonadotrophin (hCG) follow-up. Forty-one patients with complete moles, five with partial moles and one with hydropic abortion received chemotherapy for hCG elevations interpreted as persistent trophoblastic disease. Complete moles were aborted or were evacuated significantly earlier than partial moles (means of 12.1 and 15.4 weeks; P < 0.001) and hydropic abortions significantly earlier than complete moles (mean 10.7 weeks; P < 0.005). The means of the highest recorded hCG were higher in complete moles (184,056 i.v.) than in partial moles (66,259 i.v.) and hydropic abortion (7942 i.v.). When hCG became normal without chemotherapy, this occurred earlier in patients with hydropic abortion than in those with partial moles (means of 46.7 days and 62.8 days; P < 0.001) and earlier in partial moles than in complete moles (mean 78.3 days; P < 0.005). The incidence of partial moles was comparable throughout fertile years but rose to 1.9 times the average after 40 years. Complete moles were commoner between 14 and 25 years and after 35 years, reaching 4.8 times the average after 40 years. Hydropic abortions were rare before 25 years and increased with age to 12 times the average after 40 years. Stromal karyorrhexis and shape of villi, before they become hydropic, discriminate well between complete and partial mole. Hydrops increased and vascularity decreased with molar age and the presence of non-hydropic villi or vessels did not discriminate between partial mole and the younger complete moles evacuated nowadays.

Antibody-directed enzyme prodrug therapy for cancer: its theoretical basis and application.

Agents that can be administered systemically but that act selectively against cancer cells have been intensively sought but have thus far proved elusive. Nonselective cytotoxic drugs have the potential to eradicate cancer if they can be delivered selectively in sufficient concentration to cancer sites. In the approach described here, the cytotoxic agent is generated at cancer sites from a low-toxicity prodrug by the action of an enzyme delivered by an antibody to the cancer site. The feasibility of the approach has been demonstrated with a variety of enzyme-prodrug combinations.

Antibody directed enzyme prodrug therapy (ADEPT). A review of some theoretical, experimental and clinical aspects.

The concept of generating cytotoxic agents from non-toxic prodrugs at tumour sites by antibody vectored enzyme introduces a wide range of opportunities. Various prodrug-enzyme combinations have been described and encouraging results reported in xenograft models. Whilst the mouse model is a valuable tool in this approach translation to the human patient may expose more complex issues. The objective of restricting drug action to tumour sites and thus allowing greatly increased cytotoxic action requires more precise restriction of enzyme activity to tumour sites than has been achieved with an antibody vector and natural clearance alone. Assisted clearance mechanisms have been found effective. Alternatively, or additionally, the difference between prodrug and active drug creates the opportunity to degrade active drug selectively in blood and thus protect normal tissues. In order to give more than one cycle of treatment it will be necessary for the antibody-enzyme conjugate to be nonimmunogenic or for the concurrent administration of immunosuppressive agents. A pilot scale clinical trial with a prototype prodrug indicated the feasibility of antibody directed enzyme prodrug therapy (ADEPT).

Antibody-directed enzyme prodrug therapy.

A 2- or 3-stage system can overcome some of the limitations of antibody-based attempts to restrict the action of cytotoxic agents to tumour sites. These systems use the antibody to direct an enzyme, which is unique to extracellular locations in humans, to tumour sites. The tumour-located enzyme is used to activate a subsequently administered prodrug. As with any antibody-based system, the main limitation lies in the distribution of the antibody or antibody conjugate. However, in contrast to drugs or radioisotopes, an enzyme can be inactivated in nontumour tissues or subjected to rapid clearance without toxic effects. The conjugation of an enzyme to an antibody increases its immunogenicity and may require the administration of immunosuppressive agents or the development of nonimmunogenic fusion proteins. A small scale pilot clinical trial has shown the general feasibility of this approach.

The long-term outcome of patients who relapse after chemotherapy for non-seminomatous germ cell tumours.

OBJECTIVE: To examine the long-term survival of a group of patients with non-seminomatous germ cell tumours, who relapse after chemotherapy and are retreated with a cisplatin and etoposide-based regimen. PATIENTS AND METHODS: At the Charing Cross Hospital between 1979 and 1988 38 patients in relapse were seen. The median interval after primary therapy was 8 months. They were treated with an intensive cisplatin and etoposide-based regimen with cycles repeated at 7-10 day intervals, and with surgery in 22 patients. RESULTS: Forty-seven per cent of patients entered a second complete remission and 88% of these remain disease free. The overall survival was 46% with a median follow-up of more than 4.3 years. Surgery was performed in 14 of 18 patients who entered a second complete remission. Adverse risk factors before initial chemotherapy and the time to relapse did not predict outcome but patients with unresectable radiological masses after relapse therapy had a poor outcome despite normalization of serum tumour markers. The tumours of 68% of patients initially treated with cisplatin-based regimens and 62% of patients who also received etoposide remained responsive to conventional doses of these drugs at relapse. CONCLUSIONS: This study demonstrates that there is a group of patients with relapsed teratoma who can be cured without the need for very high dose chemotherapy and autologous bone marrow rescue.