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Update on Chemotherapy for Advanced Colorectal Cancer

Update on Chemotherapy for Advanced Colorectal Cancer

ABSTRACT: Efforts to improve the length and quality of life, as well as to expand treatment options, for patients with metastatic colorectal cancer have only recently become more successful. With maximization of dose and schedule schemes for fluoropyrimidine therapy, new drugs such as irinotecan (CPT-11, Camptosar) and oxaliplatin have also become part of the standard therapy for patients. Combination chemotherapy has been established to have superior response rates and progression-free survival and—in some instances, for fluorouracil and irinotecan combinations—improved overall survival compared to fluorouracil alone. There is still much to be learned about the optimal management of patients with colorectal cancer, including the role of second- and third-line chemotherapy in the overall survival outcome, and the role of salvage therapy in patients with limited metastatic disease. Most importantly, the development of a biological marker of prognosis and response should help to select appropriate chemotherapy programs for patients on a rational and individual basis, not only in the setting of metastatic disease, but also in the adjuvant population. [ONCOLOGY 15(Suppl 5):11-15, 2001]

Introduction

During the past decade, more
changes in the management of
colorectal cancer have come about than in the previous half-century of clinical
research. Such changes include the establishment of adjuvant treatment for
patients with high-risk stage II and III colorectal cancer, in addition to new
approaches to managing advanced disease. Previously, there was much nihilism in
the management of these patients. This translated into low expectations of
benefit from systemic chemotherapy or multimodality approaches, which could
possibly extend the life of a patient with metastatic disease and also possibly
cure some patients.

The past decade of clinical research in colorectal cancer has
also sparked discussion about establishing appropriate end points of clinical
benefit from chemotherapy. The gold standard for regulatory approval in the
United States is an overall survival advantage with treatment. Previously, only
the addition of leucovorin to fluorouracil (5-FU) was approved on this basis. In
addition to overall survival, many investigators think that progression-free
survival—if properly measured—could provide a surrogate of treatment effect
in clinical trials and also a measure of clinical benefit for patients.

This has not been a universally accepted end point, however, and
is not currently used for approval of oncologic drugs in the United States.
Although clinical investigators have depended on response rates to establish
interest in a treatment program, this measure is associated with wide
variability and thus may not be dependable for establishing patient benefit,
particularly in retrospective analyses. Furthermore, measures of quality of life
have generally fallen short of establishing clinical benefit of treatment. It is
in the setting of this controversy that new treatments are being assessed, at
least one of which has achieved the gold standard for approval in the United
States, namely the addition of irinotecan (CPT-11, Camptosar) to fluorouracil
(5-FU) and leucovorin for first-line therapy of patients with metastatic
colorectal cancer.

Fluoropyrimidine Therapy

Until recently, various methodologies of systemic 5-FU
administration have represented not only the standard of practice but also the
standard of care in clinical trials for patients with metastatic disease. The
development of optimal treatment programs has been both arduous and long.
Ansfield and colleagues reported a phase III trial comparing four different 5-FU
regimens in 1977.[1] Results of this early trial suggested that loading course
(ie, repetitive daily dosing) 5-FU, with moderate toxicity, was superior to a
weekly intravenous (IV) schedule, a nontoxic IV schedule, and an oral schedule.
In the latter part of the 20th century, countless patients participated in
clinical trials comparing various doses and schedules of 5-FU, with and without
new drugs.

Perhaps the most significant change in 5-FU use for metastatic
colon cancer was the development of biochemically modulated regimens with
leucovorin. Leucovorin stabilizes the ternary complex with fluorodeoxyuridine
monophosphate (FdUMP) and the enzyme thymidylate synthase. The combination of
5-FU and leucovorin has been compared with bolus 5-FU alone. A meta-analysis of
the randomized trials demonstrated a significantly improved response rate with
5-FU and leucovorin but no significant improvement in overall survival.[2]

A standard regimen for the treatment of metastatic colorectal
cancer has been the Mayo Clinic regimen, in which 5-FU and leucovorin are
administered for 5 consecutive days every 4 weeks for two cycles, followed by
the same regimen every 5 weeks. Results from the first two trials conducted at
the Mayo Clinic and the North Central Cancer Treatment Group (NCCTG) showed
response rates of 43% and 35%, and median survival times of 12.7 and 9.3 months,
respectively.[3,4] With further study, response rates with the Mayo Clinic
regimen have ranged from 11% to 19%, and median survival times from 9.2 to 13.1
months.[5-7]

Continuous-Infusion vs Bolus 5-FU

Both the efficacy and toxicity of 5-FU depend—to some degree—on
the drug dose and schedule and whether leucovorin is administered concurrently.
Early in the use of 5-FU, it was observed that the toxicity profile of
continuously infused 5-FU was different from that of bolus regimens, eg,
decreased myelosuppression and mucositis but increased dermatologic toxicities,
including hand-foot syndrome. An early clinical trial by Seifert et al comparing
continuously infused 5-FU with bolus injection demonstrated this toxicity
difference and also suggested an improved clinical benefit with infusional 5-FU
in terms of response and survival rates.[8]

A subsequent meta-analysis of six randomized trials of
continuously infused 5-FU was published in 1998.[9] Results were similar to
those obtained with the addition of leucovorin to 5-FU, with an improvement in
response rate from 14% to 22% (P = .0002), but only a marginal improvement in
overall survival from 11.3 to 12.1 months (P = .04). The incidence of
hematologic toxicity significantly decreased and hand-foot syndrome increased
with infusional 5-FU.

Many infusional 5-FU regimens are currently in use. One of the
first regimens developed is a true continuous-infusion program, in which 5-FU at
300 mg/m2 is administered for more than 4 weeks or until significant toxicity
occurs.[10] Alternative regimens have also been developed, including a high-dose
24-hour regimen known as the AIO regimen or Andolan regimen, with 5-FU doses
ranging from 2,000 to 2,600 mg/m2.[11] This regimen has also been combined with
leucovorin.

Results of a large trial comparing the 24-hour infusional
regimen, with and without leucovorin, to the Mayo Clinic regimen were recently
reported.[12] The findings suggested that the infusional approach improved
response rate and time to treatment failure, with an increase in toxicity when
leucovorin was added, but with no significant improvement in survival.

Another commonly used regimen combines bolus and infusional
administration, as devised by de Gramont and colleagues.[5] Bolus 5-FU and
leucovorin are administered on the first and second days of a 2-week cycle, with
22-hour 5-FU infusions after each bolus dose. This regimen has been compared
directly with a Mayo Clinic-type regimen in which bolus therapy was
administered every 4 weeks.[5] A significantly better response rate (32.6% vs
14.4%) and improvement in progression-free survival resulted from the infusion
approach; however, overall survival was similar with the two regimens (14.3 vs
13.1 months, P = .067).

Infusional approaches, therefore, have not consistently improved
overall survival but have been associated with higher response rates, longer
time to tumor progression, and a toxicity profile more suitable than that of
bolus regimens for combination with myelosuppressive drugs. An open question,
which will be addressed in another paper in these proceedings, is whether oral
fluoropyrimidines will offer the clinical benefit of infusional 5-FU while also
improving patient acceptance.

Based on an overview of either biochemically modulated
fluorouracil with leucovorin or methotrexate or continuous-infusion 5-FU, there
is a consistent trend toward improved response rate with newer regimens using
other than bolus 5-FU alone.[2,9] However, the impact on overall survival from
these changes has been modest, with an overall survival hazard ratio of 0.90
(0.84, 0.97) compared with bolus 5-FU alone. These data strongly suggest that
further studies of fluoropyrimidine therapy alone are unlikely to show the
clinical benefit desired for optimal management of metastatic disease.

Irinotecan

Irinotecan is a topoisomerase I inhibitor that has a mechanism
of action different from that of fluoropyrimidines. In previously treated
colorectal cancer patients, irinotecan at 125 mg/m2 weekly or 350
mg/m2 every 3
weeks resulted in response rates of 15.0% and 12.1%, respectively.[13] This
single-agent activity is comparable to that observed in patients treated with
fluoropyrimidines. In early phase II trials, the toxicity of single-agent
irinotecan included manageable myelosuppression and diarrhea. Although diarrhea
may be dose-limiting, in the clinic it is usually quite manageable with
aggressive loperamide use.

Single-Agent Activity

Based on the early phase II data and on the anticipated results
of second-line trials of irinotecan in 5-FU-refractory patients, irinotecan
was approved in the United States in 1996 for the treatment of colorectal
cancer. Approval was initially limited to 5-FU-refractory disease, but results
of two further landmark studies led to its approval as for second-line therapy
in 1998.

In the first of these trials, patients who had failed bolus 5-FU
therapy were randomly assigned to receive irinotecan plus best supportive care
or best supportive care alone.[14] The study end point was improvement in
overall survival, which was achieved. Median survival was 9.2 months in the
irinotecan-treated group and 6.5 months in
the best supportive care alone group (P = .00001). Quality-of-life measures suggested that this improvement in
survival was achieved without increasing toxicities significantly.

In the second trial, patients who had failed bolus 5-FU were
randomly assigned to receive irinotecan or one of three infusional 5-FU
regimens.[15] Median survival was 10.8 months in irinotecan-treated patients
compared with 8.5 months in fluorouracil-treated patients (P = .035). Based on
the known modest activity of infusional 5-FU in patients who have failed bolus
therapy, it was not surprising that the overall survival advantage of irinotecan
was somewhat less than in the previous study. Nevertheless, both achieved the
clinical trial objective for patients with advanced disease, which was
improvement in overall survival. Since 1998, irinotecan has been the standard
second-line therapy for 5-FU refractory patients with advanced colorectal cancer
in the United States and much of the world.

Combination Chemotherapy

Given irinotecan’s single-agent activity and its different
mechanism of action compared to 5-FU, combinations of these agents, typically
also with leucovorin, were assessed in phase I and II trials. Of several
regimens that were developed, one used bolus 5-FU and leucovorin for 4 weeks out
of 6, with irinotecan also administered on the same days.[16] In another
regimen, infusional 5-FU was administered with biweekly irinotecan
treatment.[17] Using these regimens, two phase III prospective randomized
controlled trials evaluated the combination of 5-FU, leucovorin, and irinotecan
vs 5-FU and leucovorin alone. The first of these pivotal trials compared bolus
5-FU, leucovorin, and irinotecan with bolus 5-FU and leucovorin alone.[18] A
third arm of irinotecan alone was also included.

More than 200 patients were entered into each study arm. The
primary end point was progression-free survival. Results showed that the
combination was superior in terms of response rates, time to tumor progression,
and overall survival rates.[18] Response rates were 39%, 21% , and 18% with
combination treatment, 5-FU/leucovorin, or irinotecan, respectively; this was
highly significant in favor of the combination. Time to tumor progression (7.0
vs 4.3 months; P = .004) and median survival (14.8 vs 12.6 months; P = .042)
were also significantly improved with the combination, compared with
5-FU/leucovorin alone. Once again, the available data suggest that these
improvements were achieved without a substantial decrement in overall quality of
life or increased toxicity.[19]

The second pivotal trial of combination chemotherapy with
irinotecan compared two different regimens of infusional 5-FU and leucovorin
(either the AIO or the de Gramont regimen).[20] Irinotecan was administered
weekly or biweekly according to the 5-FU schedule. This trial also demonstrated
improved response rate, time to tumor progression, and median survival for
patients receiving the irinotecan combination. Median survival, the most
important end point, was 17.4 months with the combination vs 14.1 months with
fluorouracil and leucovorin (P = .032).

A combined survival analysis of these two trials was recently
presented at the American Society of Clinical Oncology (ASCO) meeting.[21]
Combined survival for the irinotecan/5-FU/leucovorin regimen was 15.9 months,
and for the nonirinotecan regimen, 13.3 months (P = .003; survival hazard ratio,
0.79). Notably, a significant proportion of patients in both trials who were
assigned to 5-FU and leucovorin alone subsequently received second-line
irinotecan. Despite this crossover, a survival advantage was maintained,
suggesting that sequential chemotherapy for advanced colorectal cancer may not
be as effective as combination first-line treatments.

Analyses have also been undertaken to determine which patients
benefit most from combination chemotherapy.[22] Preliminary results suggest that
all patients benefit, but that those with normal performance status and lactate
dehydrogenase levels have the greatest improvement in overall survival compared
with that achieved with 5-FU and leucovorin. Preliminary results of a
cost-effectiveness analysis done in the United Kingdom also suggested that
combination chemotherapy was more cost-effective than 5-FU and leucovorin.[23]

Taken together, these data suggest that combination chemotherapy
represents a new standard of care for patients with advanced colorectal cancer.
In the United States, the combination used is 5-FU, leucovorin, and irinotecan,
as approved by the U.S. Food and Drug Administration (FDA) Oncology Drugs
Advisory Committee (ODAC) in March 2000.[19]

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