Special Feature

Novel High-Dose Chemotherapy Regimens for the Treatment of Advanced Cancer: Part II

By Daniel M. Sullivan, MD, and James S. Partyka, PharmD

Mechanisms of Resistance to Topo I and Topo II Inhibitors

Preclinical studies in our laboratory and others have defined several mechanisms of resistance to topo I and II inhibitors.1-4 This includes resistance to VP-16 and TPT, drugs we currently use in high-dose chemotherapy regimens for MM, NHL, ovarian cancer and other refractory malignancies. Drug resistance to topo II poisons may result from: 1) altered transport of the drug due to overexpression of P-glycoprotein (Pgp), multidrug resistance-associated protein (MRP), or lung resistance-related protein (LRP); 2) a mutation in the gene for topo IIa such that an enzyme with altered DNA cleavage activity is expressed;5,6 3) an attenuation of nuclear topo II content;7,8 and 4) an altered subcellular distribution of topo IIa such that it is no longer associated with the nuclear matrix9,10 or, due to a loss of its nuclear localization signal, it remains in the cytoplasm.11,12 Although the majority of studies have focused on the role of topo IIa in drug resistance, recent in vitro experiments have shown that changes in topo IIb may also be involved in drug resistance,13,14 and that the b isoform may be the preferred target of daunomycin, mitoxantrone and/or m-AMSA.13,15 The mechanisms of resistance to topo I inhibitors defined in cell lines generally parallel those of topo II with respect to a down-regulation of topo I content16,17 and inactivating mutations.18 Altered drug transport due to the overexpression of Pgp results in minimal levels of resistance to TPT19 and CPT-11/SN-38,20 while resistance to TPT21 (but not to CPT-11) may involve increased levels of MRP. A novel finding with topo I inhibitors, specifically CPT, is that the drug may be involved in regulating its own sensitivity. A continuous exposure of human KB cells to CPT has been shown to decrease topo I protein levels, decrease protein-linked DNA strand breaks, and decrease CPT cytotoxicity.22 The CPT-induced degradation of topo I is likely due to ubiquitination of the topo I population involved in a complex with DNA, which is subsequently proteolyzed by the 26S proteasome.23 TPT has also been shown to stimulate ubiquitin-mediated destruction of topo I.24 Except for AML,25 the role of alteration of topo I and/or topo II in clinical drug resistance is largely unknown.26

Clinical Studies with High-Dose Topotecan

Recently, Donato and colleagues investigated the use of escalating doses of topotecan in combination with cyclophosphamide (3 g/m2, total dose) and melphalan (140 mg/m2, total dose) followed by stem cell rescue in 22 patients (median age 45 years) with advanced ovarian cancer.27 Of the 22 patients, six patients were chemoresistant, seven patients were chemosensitive in relapse, seven patients had evidence of disease upon a second look laparotomy, and two patients were in complete remission. Topotecan was escalated from 6.25 mg/m2 to 13.75 mg/m2 (total dose) administered over five days from day-6 to day-2. Patients received peripheral blood stem cells on day 0 followed by G-CSF. Regimen-related toxicity was limited to grade 2 mucositis (n = 6) and grade 2 diarrhea (n = 2). The median day to engraftment was day +9 for an ANC greater than 500/uL and day +14 for platelets greater than 50,000/uL. Of interest, the overall response rate for patients with measurable disease was 91.7% (76% CR, 19% PR, 5% SD). The investigators concluded that although the MTD of topotecan had not been reached, the regimen was active in the treatment of advanced ovarian cancer.

Currently, our institution is completing a phase I/II high-dose topotecan study in combination with etoposide and ifosfamide.28 Topotecan has been dose-escalated from 10 mg/m2 to 64+ mg/m2 (total dose over 3 days), with stem cell rescue, in combination with ifosfamide and etoposide at fixed total doses of 10 g/m2 and 1500 mg/m2, respectively. A two-hour infusion of ifosfamide is followed immediately by a 30-minute infusion of topotecan on days -8, -7, and -6. On days -5, -4, and -3, a continuous infusion of 500 mg/m2/d etoposide was administered. We have accrued 38 patients (25 with breast cancer, 6 with NHL, 6 with ovarian cancer, 1 with testicular cancer) to this study. The maximum tolerated dose of topotecan has not been reached, although grade 3 or 4 mucositis was observed in 97% of patients. Other grade 3 or 4 regimen-related toxicities include enteritis (42%), nausea and vomiting (37%), and hematuria (8%). Since publication, there were two treatment-related deaths. One patient died on day +10 due to sepsis and cardiac failure and the other patient died on day +34 due to sepsis and CNS Aspergillus. The median day to engraftment is day +10 for an ANC greater than 500/uL and day +16 for platelets greater than 50,000/uL, untransfused. Thus, topotecan can be given in high doses in combination with an alkylating agent and a topoisomerase II inhibitor. Complete (4) and partial (9) responses have occurred in 38 patients (34%), while stable disease has been observed in 14 additional patients. The overall response rate in refractory NHL and refractory metastatic breast cancer was 67% and 31%, respectively. The overall survival and event free survival (EFS) for all patients at six months were 84 ± 7% and 41 ± 9%, respectively. (See Figures 1 and 2). RT-PCR analysis and Western analysis of patient peripheral blood lymphocytes after topotecan administration generally demonstrate a significant decrease in topo I protein levels (more than mRNA levels) and an increase in topo IIa mRNA expression. In summary, our experience at this institution suggests that high-dose topotecan in combination with ifosfamide and etoposide can result in response rates and prolonged EFS in patients with refractory cancer.

Conclusion

Determining the optimal dose and sequencing of agents in the high-dose setting is essential in the treatment of various malignancies. Recently, published data from phase II trials utilizing high-dose topotecan appear to be promising, although current studies are limited by small sample sizes and short patient follow-up. Further investigation is warranted. Therefore, at our institution we are continuing to develop phase II trials utilizing high-dose topotecan in both chemosensitive lymphoma and chemosensitive metastatic breast cancer patients. (Dr. Sullivan is Associate Professor of Medicine and Biochemistry & Molecular Biology, H. Lee Moffitt Cancer Center and Research Institute at the University of South Florida, Division of Blood and Marrow Transplantation; and Dr. Partyka is Assistant Professor of Medicine, H. Lee Moffitt Cancer Center and Research Institute at the University of South Florida, Division of Blood and Marrow Transplantation.)

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