Thioxanthene

Abstract: Phenothiazines and structurally related compounds inhibit cellular proliferation and sensitize multidrug-resistant (MDR) cells to chemotherapeutic agents. To identify more potent pharmaceuticals, we studied the structure-activity relationships of 30 phenothiazines and related compounds on cellular proliferation and MDR in sensitive MCF-7 and resistant MCF-7/DOX human breast cancer cells. Substitutions on the phenothiazine ring that increased hydrophobicity increased antiproliferative and anti-MDR activities. For example, -Cl and -CF3 groups increased whereas -OH groups decreased potency. Modifying the length of the alkyl bridge and the type of amino side chain also influenced potency. Compounds with increased activity against cellular proliferation and MDR possessed a four-carbon bridge rather than a three- or two-carbon bridge and a piperazinyl amine rather than a noncyclic amino group. Compounds with tertiary amines were better anti-MDR agents than those with secondary or primary amines but were equipotent antiproliferative agents. The effects of these substituents were unrelated to hydrophobicity. The structure-activity relationships suggest that an ideal phenothiazine structure for reversing MDR has a hydrophobic nucleus with a -CF3 ring substitution at position 2, connected by a four-carbon alkyl bridge to a para-methyl-substituted piperazinyl amine. We subsequently studied related compounds having certain of these properties. Substitution of a carbon for a nitrogen at position 10 of the tricyclic ring, with a double bond to the side chain (thioxanthene), further increased activity against MDR. For example, (trans)-flupenthixol, the most potent of these compounds, increased the potency of doxorubicin against MDR cells by 15-fold, as compared with its stereoisomer (cis)-flupenthixol (5-fold) or its phenothiazine homolog fluphenazine (3-fold). (cis)- and (trans)-flupenthixol were equipotent antiproliferative agents. (trans)-flupenthixol was not accumulated more than (cis)-flupenthixol in MDR cells, implying that their stereospecific anti-MDR effects were not the result of selective differences in the access of the drugs to intracellular targets. Both drugs increased the accumulation of doxorubicin in MDR cells, but not in sensitive cells, suggesting that they modulate MDR by interacting with a uniquely overexpressed cellular target in these resistant cells. The apparent lack of clinical toxicity of (trans)-flupenthixol makes it an attractive drug for further investigation.