PGP and ovarian carcinoma: The high-affinity GCS inhibitor, ethylenedioxy-1-phenyl-2-hexadeca-noylamino-3-pyrrolidino-1-propanol (ethylenedioxy-P4) [169], at a low dose (0.2 μM), blocked the conversion of C6-ceramide to C6-GlcCer by >90% in doxorubicin-resistant, P-gp-rich ovarian cancer cells, but that concentration did not enhance C6-ceramide cytotoxicity [164], a suggestion that inhibition of GCS is not what matters.