The camptothecin analog irinotecan(Drug information on irinotecan) (CPT-11, Camptosar) was recently approved for the treatment of fluorouracil(Drug information on fluorouracil) (5-FU)-resistant colorectal cancer, opening a new chapter in chemotherapeutic radiation sensitization. The combination of CPT-11 with irradiation can build onto the successful radiation sensitization trials with 5-FU. Both 5-FU and irinotecan have cytotoxic activity against S-phase cells; they have a defined role in the treatment of colorectal cancer;[3-5] and they are radiation sensitizers (Table 1). Radiation sensitization with these agents is dose- and schedule-dependent,[6,7] based on data from preclinical models and clinical trials. New laboratory data on the camptothecins suggest that the timing of administration may allow for dose escalation and may be an additional important factor in the design of irinotecan radiosensitizer trials.
The molecular basis for the lethal effects of ionizing radiation alone include the production of single- and double-strand breaks in DNA. Repair of x-ray and/or chemotherapy-induced genomic damage can require topoisomerase I, which is widely utilized in DNA metabolism. In the presence of camptothecin, camptothecin/topoisomerase I/DNA complexes become stabilized because the 5'-phosphoryl terminus of an enzyme-catalyzed DNA single-strand break is bound covalently to a tyrosine of topoisomerase I. Irradiation creates many single-strand breaks and these sites can 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 factor of 800 to 1,000) than its binding to undamaged supercoiled DNA.[11,12] Stabilized complexes interact with the advancing replication fork during S phase or during unscheduled DNA replication after genomic stress. The result of the presence of camptothecin is the conversion of single-strand breaks into irreversible DNA double-strand breaks, resulting in cell death.[11-13] Unrepaired DNA damage can be recognized by the p53 damage-sensing pathway, initiating and possibly amplifying the apoptotic pathway of cell death.[14,15]
Camptothecins can also modulate apoptosis independent of DNA synthesis in postmitotic neurons and confluence-arrested cell cultures, as well as in actively proliferating cell cultures and in murine tumors in vivo. High levels of topoisomerase I are associated with a high frequency of cleavable complex formation, and high topoisomerase I levels have been detected in surgical specimens from colonic, ovarian, esophageal, breast, stomach, and lung cancers, malignant melanoma, and in cultures from non-Hodgkin’s lymphoma and leukemia cells.
One basis for selective camptothecin toxicity in malignant cells as compared with normal tissues may be related to these enzyme levels. An additional biological basis for selective camptothecin activity may be related to the low pH found in cancers, which stabilizes the closed (active) lactone ring. Another effect of the camptothecin/topoisomerase I/DNA cleavable complexes is on the repair of potentially lethal damage in the DNA of plateau-phase cells.
Combined camptothecin and irradiation effects appear to be determined by position in the cell cycle. Camptothecins are considered S-phase agents because selective cytotoxicity is observed in the S phase[20,21,23] with relative sparing of G1-, G2-, and M-phase cells following pulse exposure to camptothecin. Elimination by aphidocolin of the camptothecin cytotoxicity and radiation sensitization is consistent with its S-phase effects. Additional contributions to radiation sensitization may be related to synchronizing effects of irradiation that preferentially kill G2-M cells, leaving S-phase cells intact and subject to attack by camptothecin treatment.[17,24]
Dose, Schedule, and Time of Day
In our laboratory we have examined the use of irradiation with the camptothecin analog 9-aminocamptothecin (9AC), delivered in different doses, schedules, and at different times of the day.[7,8] We determined an optimal dose and timing of irradiation with 9AC in vivo using the MCa-4 mouse mammary carcinoma. For example, 9AC given with daily fractionated irradiation resulted in dose modification factors of 2.4 (95% confidence interval [CI] = 2.0 to 3.1) and 3.7 (95% CI = 3.1 to 4.6) in animals treated with 0.5 and 2 mg/kg 9AC twice a week for 2 weeks, respectively.
In experiments in which the total dose was kept constant at 4 mg/kg, 9AC was more effective when given either twice or four times a week compared with once a week (dose modification factors of 2.8 [95% CI = 2.2 to 3.9], 2.6 [95% CI = 2.0 to 3.6], and 1.7 [95% CI = 1.3 to 2.4], respectively). The dose modification with single-dose irradiation and 9AC is markedly reduced (dose modification factor = 1.11). These results indicate that, in this murine tumor, fractionation of both the radiosensitizer dose and irradiation produces greater effect compared with large single doses used less frequently. Combined camptothecin and irradiation can also severely affect the small intestine; we have shown in the animal model that this is related to dose-dependent loss of the mucosal layer of the small intestinal villi.
We also examined the acute toxicity of 9AC administered to rodents at different times over a 24-hour period. Several phase I and II trials using chronoregulated chemotherapy have shown that drug toxicity depends on the time of day of administration, which provides strong support for a relationship between trough values of gastrointestinal epithelial cell DNA synthesis rates and reduced toxicity to anti-S-phase chemotherapeutic agents.[26-28]
Based on the hypothesis that chronomodulated delivery could be done at a time when the systemic dose would be better tolerated, we showed that the 9AC dose could be escalated by approximately 30% when administered at the time it could be best tolerated. Others have also shown that irinotecan is less toxic (less total body weight loss and acute mortality) during the animal’s resting phase when proliferation in the gastrointestinal tract is low.[29,30] These data underscore the need for a clinical trial of camptothecin as a radiosensitizer with chronomodulated administration in order to ameliorate acute toxicity.
Chemoradiation is based on cytotoxic cooperation between systemic chemotherapy and fractionated irradiation. The superiority of fractionated irradiation has long been evident because it spares late effects. When fractionated irradiation is combined with S-phase-specific agents, a form of "accelerated treatment" is produced whereby the dose-limiting toxicities are not only late morbidity of irradiation (eg, fibrosis and necrosis), but also enhanced acute toxicity expressed in the rapidly proliferating cell compartments. The pattern of dose application used in camptothecin radiation sensitization trials could thus play a significant role in success or failure.
Irinotecan is a semisynthetic derivative of camptothecin that has shown a wide range of antineoplastic activity in vitro and in vivo. Treatment schedules with irinotecan alone have varied: in the United States, 125 to 150 mg/m2 once a week for 4 weeks followed by a 2-week drug-free interval; in Europe, 350 mg/m2 once every 3 weeks; or in Japan, 100 mg/m2/wk or 150 mg/m2 every 2 weeks. Other intermittent treatment schedules using cytokine support for neutropenia, or intensive loperamide(Drug information on loperamide) for moderate to severe diarrhea, have also been reported. These irinotecan regimens in patients with colorectal cancer have resulted in median response durations ranging from 5.6 to 10.6 months, disease stabilization in 30% to 71%, and response rates of 26% to 32% in previously untreated patients.[34,35] Lower response rates have been reported for 5-FU-refractory patients (7% to 21%). Diarrhea, nausea, and vomiting are common toxicities; other side effects include asthenia, abdominal pain, leukopenia, and neutropenia.