CHEMOTHERAPY FOR UNRESECTABLE COLORECTAL CANCER:
As noted above, surgery is the only way to cure metastatic colorectal cancer. In most cases, surgery is not possible, and chemotherapy is recommended to reduce symptoms and prolong survival. Although chemotherapy provides meaningful improvements in survival, it is not possible to cure metastatic colorectal cancer with chemotherapy alone.
Continuum of care concept — With most types of incurable cancer (other than metastatic colorectal cancer), individual chemotherapy drugs or regimens are given continuously until the cancer stops responding to that drug or regimen, and then an entirely new regimen (termed "second-line therapy") may be tried.The situation is different with metastatic colorectal cancer because there are many active drugs that can be combined in a number of ways. In addition, treatment-related side effects may be lessened by limiting the number of doses of certain drugs. Thus, instead of giving the first-line regimen until the tumor progresses, treatment is often individualized.
- Specific chemotherapy drugs may be given, stopped, and then restarted at a later time, sometimes in combination with other chemotherapy drugs.
- Periods of aggressive chemotherapy may be interspersed with periods of "maintenance" chemotherapy, allowing the patient to have the greatest possible quality of life while minimizing side effects.
This has been referred to as the "continuum of care" approach to treatment of metastatic colorectal cancer.
Conventional chemotherapy — The conventional chemotherapy drugs used to treat metastatic colorectal cancer include:
- 5-fluorouracil (abbreviated FU), which is usually given into the vein with a second drug called leucovorin, which enhances its activity
- Orally active FU-like drugs, such as capecitabine (Xeloda®)
- Oxaliplatin (Eloxatin®), which is given intravenously
- Irinotecan (Camptosar®), also given intravenously
These drugs work by interfering with the ability of rapidly growing cells (like cancer cells) to divide or reproduce themselves. Because most of an adult's normal cells are not actively growing, they are less affected by chemotherapy, with the exception of bone marrow (where the blood cells are produced), the hair, and the lining of the gastrointestinal tract. Effects of chemotherapy on these and other normal tissues cause side effects during treatment.
Targeted chemotherapy — Three other drugs that are active against metastatic colorectal cancer work by a different mechanism. These are referred to as "targeted chemotherapy agents" since they are antibodies (a type of protein) that work to inhibit a specific protein that is important for the growth and/or survival of colon cancer cells.
Because targeted chemotherapy does not directly interfere with rapidly dividing cells, they do not have the usual side effects of conventional chemotherapy. However, targeted chemotherapy has other unique side effects, which are described in detail below.
Currently available targeted chemotherapy includes:
Bevacizumab (Avastin®) — Bevacizumab binds to a protein called vascular endothelial growth factor (VEGF). VEGF is involved in the development of a blood supply within a growing cancer; this blood supply is essential for the tumor to grow and spread. Bevacizumab enhances the antitumor effect of other chemotherapy drugs. Bevacizumab is not effective when given by itself, but is generally given in combination with other drugs, such as oxaliplatin and irinotecan
Cetuximab (Erbitux®) — Cetuximab targets a different protein, the epidermal growth factor receptor (EGFR), which is found in about 80 percent of colorectal cancers. Erbitux® is effective even if EGFR is not found in an individual tumor.Cetuximab does not work for all patients. It depends on whether or not the tumor has a specific abnormality (a mutation in a gene called K-ras).
- If the tumor has a K-ras mutation, cetuximab does not work.
- If the tumor does not have a K-ras mutation, cetuximab does work (ie, it is effective).
Unlike bevacizumab, cetuximab is active when given alone or in combination with other drugs, like irinotecan.
Panitumumab (Vectibix®) — Like cetuximab, panitumumab also targets the EGFR. Like cetuximab, it is effective only for tumors that do not have a specific mutation in the K-ras gene.
Monitoring during treatment — A person's response to chemotherapy is monitored with periodic X-ray studies (such as CT scans) every six to eight weeks during therapy. In addition, blood levels of a tumor marker called carcinoembryonic antigen (CEA) are generally measured every one to three months during therapy. CEA levels are typically high in people with advanced colorectal cancer; persistently rising CEA levels suggest that disease is progressing and a change in therapy is warranted.
However, a rising CEA alone is not sufficient evidence to prompt a change in treatment. Disease progression should be confirmed with radiographic testing (eg, CT scan) or a biopsy before changing treatment.
Tony Talebi, MD