Camptothecin Radiation Sensitization: Mechanisms, Schedules, and Timing

Introduction

Recent Food and Drug Administration approval of the camptothecin analog irinotecan(Drug information on irinotecan) (CPT-11 [Camptosar]) for the treatment of colorectal cancer resistant to fluorouracil(Drug information on fluorouracil) (5-FU)[1] has opened a new chapter in chemotherapeutic radiation sensitization. High interest in using this and other camptothecins (eg, topotecan(Drug information on topotecan) [Hycamtin], 9-aminocamptothecin) in combination with irradiation is based in part on past successes with 5-FU, an antimetabolite, as a radiation sensitizer.[2] Both camptothecin analogs and antimetabolites have cytotoxic activity against S-phase cells, and both have a defined role in the treatment of colorectal cancer, a disease in which radiation sensitization has improved locoregional control and overall survival.[3]

Radiation sensitization with either class of agents is dose- and schedule-dependent,[4] and the importance of the timing of administration of these drugs when given with fractionated irradiation is a new factor that is gaining attention. This knowledge combined with new laboratory data will be important in the design of new camptothecin radiosensitizer trials.

Cytotoxic Mechanisms of Camptothecins and Irradiation

The molecular basis for the lethal effects of ionizing radiation alone include the production of single- and double-strand breaks in DNA.[5] Another basic observation regarding repair of x-ray- or chemotherapeutic-induced genomic damage is the requirement for topoisomerase I, which is widely used in DNA metabolism.[6]

In the presence of camptothecin, the camptothecin-topoisomerase I-DNA complex becomes stabilized because the 5¢-phosphoryl terminus of an enzyme-catalyzed DNA single-strand break is bound covalently to a tyrosine of topoisomerase I. Irradiation can create thousands of single-strand breaks per cell per gray, leaving these sites to be attacked by topoisomerase I in the presence of camptothecin. The rate of topoisomerase I binding to nicked DNA is also more rapid (increased by a factor of 800 to 1,000) than its binding rate to undamaged supercoiled DNA.[7,8] These stabilized complexes interact with the advancing replication fork during S-phase or during unscheduled DNA replication after genomic stress and cause the conversion of single-strand breaks into irreversible DNA double-strand breaks, resulting in cell death (Figure 1).[7,9]

Topoisomerase I may also compete with DNA repair complexes (DNA ligases, poly-(adenosine diphosphate)-ribosyl-transferase) for the single-strand breaks. In the presence of camptothecins, this can result in unrepaired DNA damage that can be recognized by the p53 damage-sensing pathway, initiating and possibly amplifying the apoptotic pathway of cell death.[7,10-12] The camptothecins have also been found to modulate apoptosis independently of DNA synthesis in postmitotic neurons[13] and confluent cell cultures,[10] as well as in actively proliferating cell cultures[14] and in murine tumors in vivo.[15]

High levels of topoisomerase I are associated with a high frequency of cleavable complex formation.[16] High topoisomerase I levels have been detected in surgical specimens from malignant melanoma, colonic, ovarian, esophageal, breast, stomach, and lung cancers, and in cultures from non-Hodgkin’s lymphoma and leukemia cells.[17] One basis for selective camptothecin toxicity in malignant cells compared to normal tissues may relate to these enzyme levels. An additional biological basis for selective camptothecin activity is the low pH found in cancers that can stabilize the closed (active) lactone ring form.[17,18]

In addition, DNA-topoisomerase I-camptothecin cleavable complexes affect the repair of potentially lethal damage in plateau phase cells (Figure 1).[7] In contrast, in log phase cells, lethality caused by camptothecin and irradiation also appears to be determined by effects on the cell cycle; ie, there appears to be differential phase specificity of cell killing by drug and irradiation. The camptothecins are considered to be S-phase agents since selective cytotoxicity is observed in S-phase,[14,16,18,19] while G₁-, G₂-, and M-phase cells are relatively spared following pulse exposure to camptothecin.[19] Elimination by aphidocolin of camptothecin-induced cytotoxicity and radiation sensitization is consistent with these S-phase-selective effects.[20]

An additional contribution to radiation sensitization by combination treatment may be the synchronizing effect of irradiation itself that preferentially kills G₂- through M-cells, thus leaving S-phase cells intact and subject to attack by camptothecin.[14,19]

Clinical Studies

The clinical basis for chemoradiation is cytotoxic cooperation between systemic chemotherapy and irradiation when chemotherapeutic drugs are given concurrently with fractionated irradiation. Clinical research has established the superiority of fractionated irradiation, which spares late toxic effects. When S-phase-specific agents are administered with conventional irradiation (eg, 2 Gy/d), this regimen becomes a form of accelerated treatment.[21] With such regimens, the dose-limiting toxicity may consist not only of the late morbidity of irradiation (eg, fibrosis and necrosis) but also enhanced acute toxicity expressed in the rapidly proliferating cell compartments. Thus, the pattern of dose application used in camptothecin radiation sensitization studies will likely play a role in their success or failure.

Topotecan

Topotecan is a water-soluble topoisomerase I inhibitor with cytotoxic activity in a variety of preclinical models. Topotecan exhibits schedule-dependency in vivo and has high cytotoxic activity with frequently repeated (daily) dosing schedules.[22,23]

In murine systems, there is evidence that reducing the dose intensity (by prolonging the drug administration schedule using a small amount with each treatment) provides a therapeutic advantage because of reduced host toxicity and equal or superior tumor responses. In clinical studies, the short plasma half-life of topotecan also suggests that prolonged drug exposure by infusion could be effective.[24] In a phase I trial, an escalating low-dose topotecan infusion was found to have an increased therapeutic ratio when compared to an intermittent dosing schedule.[24] Neutropenia is usually the dose-limiting toxicity of topotecan.

Phase II studies have shown that topotecan alone has cytotoxic activity in lung cancer with intermittent (daily × 5 every 21 days) dosing schedules,[25] as well as in lung cancer patients with topoisomerase II-refractory disease.[26] In advanced head and neck cancer patients, topotecan is well-tolerated and has single-agent activity similar to that of cisplatin (Platinol), 5-FU, and methotrexate(Drug information on methotrexate).[27]

Decreased production or mutation of topoisomerase I can cause resistance to the cytotoxic effects of topotecan and other camptothecins. Active efflux of the camptothecins by P-glycoprotein-mediated transport may also contribute to resistance.[28]

Radiation Sensitization--Topotecan has demonstrated radiation-sensitizing properties in log and plateau phase cell cultures[29,30] and in murine fibrosarcomas in vivo.[31,32] Clinical trials have begun in patients with non-small-cell lung cancer and in patients with central nervous system tumors.

Clinical evidence of radiation sensitization with topotecan has been demonstrated in a dose-escalation trial in patients with locally advanced, inoperable non-small-cell lung cancer.[33]. In this trial, 12 patients received 60 Gy (2 Gy/d) of radiation plus topotecan delivered by bolus injection on days 1 through 5 and on days 22 through 26, beginning on the same day as irradiation. The initial dose level of topotecan was 0.5 mg/m²; dose levels of 0.75 and 1.0 mg/m² were also tested. Doses higher than 0.5 mg/m² were associated with relatively high acute hematologic and gastrointestinal toxicity.

Of the 12 patients, 5 survived (2 without evident disease) and 7 died of their cancer. Severe late pulmonary toxicity was not reported, but pneumonitis was noted.

The Radiation Therapy Oncology Group (RTOG) is evaluating topotecan plus cranial irradiation in patients with glioblastoma multiforme. The Children’s Cancer Group (CCG) is also evaluating this combination in children with pontine gliomas. In both of these trials, topotecan is given daily as a 30-minute infusion 30 to 120 minutes before irradiation.