The present invention relates to 5-amino-3-phenylamino-6-phenylpyrazolo[3,4-d]pyrimidine derivatives of the general formula (I) or pharmaceutically acceptable salts or propharmacons thereof for use as BCRP inhibitors, wherein at least one hydrogen atom in at least one of the phenyl groups A and B is substituted by a substituent RH, which has a Hammett constant σp greater than 0.23. For corresponding compounds, surprisingly a particularly high inhibitory activity against BCRP has been discovered which can be exploited for suppressing the multidrug resistance modulator BCRP, thus providing an improvement in efficacy of BCRP affected drugs. This has useful implications for cancer and HIV treatment.
