But the mutations apparently cause kras to become permanently stuck in the "on" position.
Amgen has published a big study showing that its Vectibix only works in patients with normal kras.
Already, big hospitals like Memorial Sloan-Kettering are gearing up to do kras testing on all their colon patients.
Few gene-testing studies yield the sort of clear-cut results seen in the case of kras, Erbitux and Vectibix.
Avastin doesn't cause the facial rash and appears to work equally well in those with or without kras mutations.
If there is any benefit of the drugs in those with kras mutations, "it is exceedingly small, " he adds.
These patients have tumors with a mutation in a growth-promoting gene called kras that may render the drugs ineffective.
He predicts that the finding could expand the use of Erbitux in patients who do not have a kras mutation.
ImClone is now waiting for more data from this study to look at whether patient with normal kras also lived longer.
But UCLA's Jonsson Cancer Center is also in the process of gearing up to perform the kras gene test, says J.
Erbitux and Vectibix are antibodies that bind to receptors on the cell surface called EGF and blocks signals that turn kras on.
The Erbitux-kras finding appears to be rock solid, despite its retrospective nature.
In Europe, Vectibix is only approved for patients who have normal kras.
There is no proof that using Erbitux up front can keep people alive longer than using Avastin, even in those with normal kras.
But based on the new kras findings, "You will likely see Erbitux moved up" and used sooner in patients with normal kras, predicts ImClone Chief Executive John H.
Much more data on kras will be presented at a big meeting of cancer doctors in Chicago at the beginning of June that could clarify the matter once and for all.
But when the researchers reanalyzed the study looking just at those patients with normal kras, they found that in this group Erbitux delayed the disease progression by a much more robust 32%.
He predicts that very soon all patients with colon cancer will be tested early on for kras status in the same way the breast cancer patients are tested to see their estrogen receptor status.
In particular, researchers will re-analyze a previous big trial of Erbitux as initial colon cancer treatment to see if the drug combo worked better in the subset of patients who don't have kras mutations.
Since there is no evidence that holding off on Erbitux in the 60% of patients with normal kras will harm them, he argues that doctors should wait until they really need to use the drug.
One French study published in the Jan. 20 Journal of Clinical Oncology found that none of the 24 patients with kras mutation responded to Erbitux, while tumors shrank in 40% of those with normal kras.
"We can now say scientifically there is no reasonable chance it will help" people with kras mutations, says Leonard Saltz, of Memorial Sloan-Kettering Cancer Center, who predicts that insurance companies will soon require a kras test for colon patients.
The emerging data, much of it being released at a big meeting of cancer specialists this weekend in Chicago, reveal that Erbitux is utterly ineffective in about 40% of colon cancer patients whose tumors have a mutation in a gene called kras.
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