Lack of enantio-selectivity in the in vitro antitumour cytotoxicity and membrane-damaging activity of ether lipid SRI 62-834: further evidence for a non-receptor-mediated mechanism of action.
SRI 62-834 ([tetrahydro-2-(octadecycloxy)methylfuran- 2-yl]methoxylphosphocholine; CRC 86-05; NSC 614383) is a cyclic antitumour ether lipid (AEL) with a novel, but ill-defined, mechanism of action. AELs are believed to act on membranes and cell signals, but the precise mechanisms of selectivity are unclear. Receptor-mediated mechanisms can often be identified by the differential activity of the individual stereoisomers of a drug. We have therefore compared the R- and S-enantiomers of SRI 62-834 for: (1) cytotoxicity against the human HT29 colon carcinoma cell line using a tetrazolium dye reduction assay and (2) membrane-damaging effects monitored by 51Cr radiolabel release. The tetrazolium assay revealed near-identical mean ID50 values around of 2-3 microM for the R- and S-isomers as well as for the racemic mixture. Moreover, pre- and co-incubation of the cells with the potent platelet-activating factor (1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine;PAF) receptor antagonist WEB 2086BS (3-[4-(chorophenyl)-9-methyl-6H-thieno[3,2- f][1,2,4]triazolo-[4,3-a][1,4]-diazepin-2-yl]-1-(4- morpholinyl)-1-propanone) had no effect on the cytotoxicity of either isomer or the racemate. Short-term membrane damage was not evident at low micromolar concentrations and between 139 and 163 microM either lipid was required to release 50% of the incorporated 51Cr label. Again, there was no difference in potency between the enantiomers and the racemate. Coincubation with WEB 2086BS also failed to modulate the membrane-lytic potency of the AELs. These results indicate that the site(s) of cytotoxic action of SRI 62-834 is (are) not stereospecific and also appear to rule out the involvement of a conventional PAF receptor in the mechanism of action of SRI 62-834.
Lohmeyer
M
M
MRC Clinical Oncology and Radiotherapeutics Unit, MRC Centre, Cambridge, U.K.
Workman
P
P
D000970:Antineoplastic Agents / Q000494:pharmacology*; D001381:Azepines / Q000494:pharmacology; D016923:Cell Death / Q000187:drug effects; D002462:Cell Membrane / Q000187:drug effects; D005663:Furans / Q000494:pharmacology*; D006801:Humans; D010742:Phospholipid Ethers / Q000494:pharmacology*; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology; D013237:Stereoisomerism; D013778:Tetrazolium Salts; D013844:Thiazoles; D014230:Triazoles / Q000494:pharmacology; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000473:pathology
D000970:Antineoplastic Agents; D001381:Azepines; D005663:Furans; D010742:Phospholipid Ethers; D010975:Platelet Aggregation Inhibitors; D013778:Tetrazolium Salts; D013844:Thiazoles; D014230:Triazoles; C052518:WEB 2086; C055462:SRI 62-834; C022616:thiazolyl blue