market share in the third quarter of 2005 (the end of the sample period), and regimens with missing attribute data.
Market shares for the 12 regimens in our sample and the outside option are plotted in Figure 1. The regimens are also described more fully in Table 1, arranged in order of entry. Between 1993 and 1996, about 95 percent of colorec- tal cancer patients were treated with 5-FU/leucovorin, which at that time was generic, with the remainder treated with off-label drugs or regimens with very small market share.12 Irinotecan (brand name Camptosar) was approved by the FDA for treating colorectal cancer in 1996, and over the next several years the market share of irinotecan (approved as a second-line treatment for patients who had already been treated with a different chemotherapy regimen) and irinote- can combined with 5-FU/LV grew at the expense of 5-FU/LV.13 Capecitabine (Xeloda), a tablet that produces the same chemical response as 5-FU/LV, was approved for treatment of colorectal cancer in April of 2001 and was adminis- tered as a standalone therapy or combined with irinotecan. All other drugs for treating colorectal cancer in our sample are delivered intravenously (IV) under the supervision of a physician or nurse.
Oxaliplatin (Eloxatin) was introduced in August of 2002, followed by cetux- imab (Erbitux) and bevacizumab (Avastin) in February of 2004. By the third quarter of 2005, two of the regimens created by these three new drugs (oxali- platin + 5-FU/LV; and bevacizumab + oxaliplatin + 5-FU/LV) surpassed the market share of 5-FU/LV, whose share had fallen to about 14 percent.
The market shares of several regimens change sharply in the first quarter of 2002 when we use market share data from IntrinsiQ rather than SEER. One explanation for these changes is that Medicare patients may be treated with different regimens than non-Medicare patients. Another possible explanation is
5-FU contains the drug fluorororacil. Because it takes Medicare a while to code new drugs into their proper NDC code, for several quarters a
new drug will appear in the outside option.