Although Karlb's solution will work, it tends to be slow. An alternative solution is to transform the data from wide form to long form. The new form of the data will contain 600 BY groups. The first contains the data for Y, Drug, and X1. The second contains the data for Y, Drug, and X2. And so on. For an example and discussion, see the article
"An easy way to run thousands of regressions in SAS"
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