Jun 13, 2002 - 10:35 pm
Drug resistance is the principal reason for the failure of chemotherapy. Some tumors (like adenocarcinoma) are highly resistant to virtually all anti-cancer drugs, while other tumors are quite sensitive. Most patient tumors fall in between these two extremes, with resistance to some drugs but not to others. Many mechanisms by which cancer cells develop resistance have been identified. Most cancers have some degree of drug resistance with many patients not responding to the chemotherapy most commonly prescribed for the tumor type.
The results of years of clinical trials on patient populations are considered enough indication on how an individual will respond. The percentage of patients that must respond to a drug before it is approved varies from as low as 20% to as high as 80%, depending on the type of cancer. Thereafter it is used routinely for all patients with the same form of cancer, though unfortunately a drug that helps one person does not necessarily mean that it will help all people with the same diagnosis.
The notion that patients treated by conventional therapies live longer than untreated victims is biased by the methods of defining the groups studied. If a person in the untreated category dies at any time while he or she is being studied, this is recorded as a death in the control group and registered as a failure of the no treatment approach. However, if patients in the treated category die during the course of treatment (before the course is completed), their cases are rejected from the data since these patients do not then meet the criteria established by definition of the term "treated". A patient dying on day 89 of a prescribed 90-day course of chemotherapy would be dropped from the list of treated patients.
Since both radiation and chemotherapy suppress the immune system, it is possible that new tumors are allowed to grow because the patient has been rendered unable to resist them. A person who is cured of cancer by these drastic means may find himself struggling with a new, drug-induced tumor a few years later. A malfunctioning immune system can fail to stop the growth of cancer cells.
The chemotherapeutic drug Taxol is heavily used as a cocktail with Carboplatin. The effectiveness of the cocktail regimen is limited because ovarian cancer is ordinarily detected in its late stages (as lung and breast cancer can also) and most patients develop resistance to chemotherapy drugs. Virtually all cancers contain cells resistant to chemotherapy and the number of drug resistant cells increases expodentially as a tumor expands in size. After treatment is completed the surviving drug resistant cells often proliferate rapidly.