The present invention relates to the use of a combination of azathioprine (AZA) and L- buthionine-[S,R]-sulfoximine (BSO), or the pharmaceutically suitable salts thereof, in the preparation of a drug useful in the treatment of a refractory tumor, preferably cancer. The combination in the invention has been shown to be especially effective, in particular in the treatment of solid tumors of the liver, colon and colon metastasis to the liver, tumors that are highly resistant to chemotherapy. The invention likewise relates to the pharmaceutical composition containing the combination and to the kit containing same.