The camptothecins are a new class of chemotherapeutic radiation sensitizers. Clinical trials with camptothecins alone show higher toxicity than predicted by preclinical models, which has created the challenge of finding new
ABSTRACT: The camptothecins are a new class of chemotherapeutic radiationsensitizers. Clinical trials with camptothecins alone show higher toxicity thanpredicted by preclinical models, which has created the challenge of finding newways to widen the therapeutic window. Camptothecin dose, schedule, and timingwith irradiation are important factors that need to be considered in the designof new studies with these S-phase agents. Data are reviewed from early phase Iand II chemoradiation trials, including a multicenter, phase II study planned bythe Radiation Therapy Oncology Group (RTOG) in operable rectal cancer usingirinotecan (CPT-11, Camptosar). One novel approach (based on preclinicalobservations) with the potential to widen the therapeutic window may be the useof a chronomodulated camptothecin delivery schedule with irradiation. [ONCOLOGY15(Suppl 5):37-41, 2001]
The camptothecin analog irinotecan (CPT-11, Camptosar) wasrecently approved for the treatment of fluorouracil (5-FU)-resistant colorectalcancer, opening a new chapter in chemotherapeutic radiation sensitization.The combination of CPT-11 with irradiation can build onto the successfulradiation sensitization trials with 5-FU. Both 5-FU and irinotecan havecytotoxic activity against S-phase cells; they have a defined role in thetreatment of colorectal cancer;[3-5] and they are radiation sensitizers (Table1). Radiation sensitization with these agents is dose- andschedule-dependent,[6,7] based on data from preclinical models and clinicaltrials. New laboratory data on the camptothecins suggest that the timing ofadministration may allow for dose escalation and may be an additional importantfactor in the design of irinotecan radiosensitizer trials.
The molecular basis for the lethal effects of ionizing radiationalone include the production of single- and double-strand breaks in DNA.Repair of x-ray and/or chemotherapy-induced genomic damage can requiretopoisomerase I, which is widely utilized in DNA metabolism. In the presenceof camptothecin, camptothecin/topoisomerase I/DNA complexes become stabilizedbecause the 5'-phosphoryl terminus of an enzyme-catalyzed DNA single-strandbreak is bound covalently to a tyrosine of topoisomerase I. Irradiation createsmany single-strand breaks and these sites can be attacked by topoisomerase I inthe presence of camptothecin. The rate of topoisomerase I binding to nicked DNAis also more rapid (increased by factor of 800 to 1,000) than its binding toundamaged supercoiled DNA.[11,12] Stabilized complexes interact with theadvancing replication fork during S phase or during unscheduled DNA replicationafter genomic stress. The result of the presence of camptothecin is theconversion of single-strand breaks into irreversible DNA double-strand breaks,resulting in cell death.[11-13] Unrepaired DNA damage can be recognized by thep53 damage-sensing pathway, initiating and possibly amplifying the apoptoticpathway of cell death.[14,15]
Camptothecins can also modulate apoptosis independent of DNAsynthesis in postmitotic neurons and confluence-arrested cell cultures,as well as in actively proliferating cell cultures and in murine tumors invivo. High levels of topoisomerase I are associated with a high frequency ofcleavable complex formation, and high topoisomerase I levels have beendetected in surgical specimens from colonic, ovarian, esophageal, breast,stomach, and lung cancers, malignant melanoma, and in cultures from non-Hodgkin’slymphoma and leukemia cells.
One basis for selective camptothecin toxicity in malignant cellsas compared with normal tissues may be related to these enzyme levels. Anadditional biological basis for selective camptothecin activity may be relatedto the low pH found in cancers, which stabilizes the closed (active) lactonering. Another effect of the camptothecin/topoisomerase I/DNA cleavablecomplexes is on the repair of potentially lethal damage in the DNA ofplateau-phase cells.
Combined camptothecin and irradiation effects appear to bedetermined by position in the cell cycle. Camptothecins are considered S-phaseagents because selective cytotoxicity is observed in the S phase[20,21,23] withrelative sparing of G1-, G2-, and M-phase cells following pulse exposure tocamptothecin. Elimination by aphidocolin of the camptothecin cytotoxicityand radiation sensitization is consistent with its S-phase effects.Additional contributions to radiation sensitization may be related tosynchronizing effects of irradiation that preferentially kill G2-M cells,leaving S-phase cells intact and subject to attack by camptothecintreatment.[17,24]
Dose, Schedule, and Time of Day
In our laboratory we have examined the use of irradiation withthe camptothecin analog 9-aminocamptothecin (9AC), delivered in different doses,schedules, and at different times of the day.[7,8] We determined an optimal doseand timing of irradiation with 9AC in vivo using the MCa-4 mouse mammarycarcinoma. For example, 9AC given with daily fractionated irradiationresulted in dose modification factors of 2.4 (95% confidence interval [CI] = 2.0to 3.1) and 3.7 (95% CI = 3.1 to 4.6) in animals treated with 0.5 and 2 mg/kg9AC twice a week for 2 weeks, respectively.
In experiments in which the total dose was kept constant at 4mg/kg, 9AC was more effective when given either twice or four times a weekcompared with once a week (dose modification factors of 2.8 [95% CI = 2.2 to3.9], 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 murinetumor, fractionation of both the radiosensitizer dose and irradiation producesgreater effect compared with large single doses used less frequently. Combinedcamptothecin and irradiation can also severely affect the small intestine; wehave shown in the animal model that this is related to dose-dependent loss ofthe mucosal layer of the small intestinal villi.
We also examined the acute toxicity of 9AC administered torodents at different times over a 24-hour period. Several phase I and II trialsusing chronoregulated chemotherapy have shown that drug toxicity depends on thetime of day of administration, which provides strong support for a relationshipbetween trough values of gastrointestinal epithelial cell DNA synthesis ratesand reduced toxicity to anti-S-phase chemotherapeutic agents.[26-28]
Based on the hypothesis that chronomodulated delivery could bedone at a time when the systemic dose would be better tolerated, we showed thatthe 9AC dose could be escalated by approximately 30% when administered at thetime it could be best tolerated. Others have also shown that irinotecan isless toxic (less total body weight loss and acute mortality) during the animal’sresting phase when proliferation in the gastrointestinal tract is low.[29,30]These data underscore the need for a clinical trial of camptothecin as aradiosensitizer with chronomodulated administration in order to ameliorate acutetoxicity.
Chemoradiation is based on cytotoxic cooperation betweensystemic chemotherapy and fractionated irradiation. The superiority offractionated irradiation has long been evident because it spares late effects.When fractionated irradiation is combined with S-phase-specific agents, a formof "accelerated treatment" is produced whereby the dose-limitingtoxicities are not only late morbidity of irradiation (eg, fibrosis andnecrosis), but also enhanced acute toxicity expressed in the rapidlyproliferating cell compartments. The pattern of dose application used incamptothecin radiation sensitization trials could thus play a significant rolein success or failure.
Irinotecan is a semisynthetic derivative of camptothecin thathas shown a wide range of antineoplastic activity in vitro and in vivo.Treatment schedules with irinotecan alone have varied: in the United States, 125to 150 mg/m2 once a week for 4 weeks followed by a 2-week drug-free interval; inEurope, 350 mg/m2 once every 3 weeks; or in Japan, 100 mg/m2/wk or 150mg/m2every 2 weeks. Other intermittent treatment schedules using cytokine supportfor neutropenia, or intensive loperamide for moderate to severe diarrhea, havealso been reported. These irinotecan regimens in patients with colorectalcancer have resulted in median response durations ranging from 5.6 to 10.6months, disease stabilization in 30% to 71%, and response rates of 26% to32% in previously untreated patients.[34,35] Lower response rates have beenreported for 5-FU-refractory patients (7% to 21%). Diarrhea, nausea, andvomiting are common toxicities; other side effects include asthenia, abdominalpain, leukopenia, and neutropenia.
US and Japanese Trials
The US trials showed that more than 50% of patients had at leastone adverse event. Grade 3 or 4 toxicity occurred in about one-third ofpatients, and was most commonly late diarrhea or neutropenia. The mechanismunderlying gastrointestinal toxicity after single or repeated daily irinotecantreatment is determined by the S-phase specificity of camptothecins in rapidlyproliferating cell populations like the intestinal mucosa and bone marrow.Biliary excretion of camptothecin conjugated with glucuronic acid is animportant elimination route of the drug. Glucuronidase of intestinalmicroflora can cleave the excreted camptothecin-glucuronide, releasing free drugin the intestinal lumen,[36,37] further increasing the potential fordiarrhea.
Irinotecan has significant activity in non-small-cell lungcancer, and has been combined with radiation in phase I/II trials in Japan.Irinotecan, a prodrug, must first be converted to its active form SN-38 bycarboxylesterase, an enzyme that has been found in liver, blood, and lung cancerbiopsies. Carboxylesterase was detected by immunostaining with an antihumancarboxylesterase polyclonal antibody and by indirect immunostaining in lungsquamous cell carcinomas, which had significantly higher levels thanadenocarcinoma cells (P < .05). Other studies in 10 human lung cancer celllines showed that SN-38 levels increased significantly over 24 hours, suggestingthat human lung cancer cells efficiently convert irinotecan to SN-38.
Radiation sensitization with irinotecan in two human lung cancerxenografts has been reported, where irinotecan was administered in nontoxicdoses 1 hour prior to a single dose of irradiation. Other reports show thatradiation sensitization with irinotecan occurs during or after irradiation.
Phase I trials with irinotecan and concurrent irradiation (60 Gyin 30 fractions over 6 weeks) have been reported for non-small-cell lungcancer. Doses of 40 to 60 mg/m2 were used.[43-48] A maximum tolerated dose of 40mg/m2 (by 90-minute IV infusion) given weekly for 6 weeks was found in theVanderbilt Cancer Center Affiliate Network trial, while Japaneseinvestigators have used doses as high as 60 mg/m2 with higher toxicityrates.[43-47] Overall objective response rates ranged from approximately 50% to79%, and acute toxicities were leukopenia, neutropenia, hypoxemia due topneumonitis, esophagitis, fever, and diarrhea. The combination of weeklyirinotecan and concurrent radiation therapy is thus an active treatment forlocally advanced non-small-cell lung cancer, but a high incidence ofpneumonitis requires caution.
Our proposed phase II trial of chronomodulated weekly irinotecanplus thoracic irradiation is shown in Figure1. This approach is feasible withthe use of programmable pumps, which have already been used in chemoradiationtrials with infusional 5-FU. This study is designed to evaluate the activity andtolerability of irinotecan at 60 mg/m2/wk given at the time of day it would bebest tolerated. Preliminary human data suggest that irinotecan is bettertolerated when given at 5:00 am than at "standard" times later in theday.
Irinotecan plus irradiation for patients with advanced orrecurrent colorectal cancer is also being studied. In a phase I study, a singleweekly irinotecan dose of 50 mg/m2 is being combined with protracted infusional5-FU (225 mg/m2/d) and concurrent pelvic radiotherapy of 45 Gy. Thetoxicities associated with this treatment are gastrointestinal andmyleosuppression. The pathologic complete response rate for 20 patients withadvanced and recurrent rectal cancer is 25%, which is higher than that usuallyassociated with this disease status (ie, less than 15%). This suggests that thiscombination of chemoradiation provides enhanced antitumor activity; this programis being explored further by the Radiation Therapy Oncology Group (RTOG) in arandomized phase II study (RTOG-R-0012) (Figure2). This approach builds onto anexisting standard practice of infusional chemoradiation for operable rectalcancer.
Daily Low-Dose Irinotecan
Another chemoradiation approach being used is based on phase Idata with daily low-dose irinotecan. The rationale is thatcamptothecin-stabilized DNA/topoisomerase I cleavable complexes are reversible,and thus frequent irinotecan dosing may favor production of these complexes.This is supported by the fact that the half-life of SN-38 is relatively long anddaily bolus injections can result in a concentration-time product similar tothat produced with a continuous infusion. Irinotecan was administered in a phaseI fashion for 5 consecutive days for 2 weeks followed by a 1-week rest.
In 20 previously treated patients with advanced tumors (16 withcolorectal cancer), acute toxicity over the first two cycles (6 weeks’duration) was mild diarrhea and neutropenia. Two patients with colorectal cancerachieved a partial response and six others had stable disease. Grade 3 diarrheaand neutropenic fever were seen at the highest dose level (22 mg/m2/d), and doseescalation was stopped. This treatment scheme delivered 88% of the amount ofdrug given on a more standard weekly × 4 schedule. The investigators thoughtthat less heavily pretreated patients might tolerate higher doses.
A divided dose bolus administration schedule of irinotecan(Monday through Friday, weeks 1, 2, 4, and 5) during a course of irradiation(50.4-Gy dose) has been used for preoperative treatment of locally advanced orunresectable rectal cancer. Irinotecan doses were escalated in a phase Istudy of 17 patients; two of three patients at 13 mg/m2/d had dose-limitingdiarrhea. The maximum tolerated dose was 10 mg/m2/d and additional patients havebeen evaluated at this dose level. Diarrhea and neutropenia have beendose-limiting toxicities. All patients had complete resection of their rectaltumors. This combined-modality schedule of daily irinotecan and irradiationappears to be well tolerated and a larger phase I/II trial is planned todetermine tolerability and efficacy.
Irinotecan is being actively investigated as a radiationsensitizer. Evidence from murine solid tumor models indicates that an optimalschedule of administration and timing of administration of irinotecan couldwiden the therapeutic window. Schedules using repeated dosing of thecamptothecins show good efficacy in murine models and are based on cytokineticand pharmacologic data. The repeated dose schedule, in turn, is consistent witha radiobiologic basis for fractionation with irradiation. New evidence alsosuggests that optimal timing of irinotecan with daily irradiation could lead tofurther gains by reducing normal tissue toxicity. These ideas await confirmationin prospective clinical trials.
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