Abstract
Tachyphylaxis may have contributed to the failure of the motilide ABT-229 [N-ethyl, N-methyl 4′′ deoxy erythromycin (EM)-B enolether] in clinical trials. We compared the desensitizing potency of structurally related motilides [EM-A, EM-A enolether (ME4), N-ethyl, N-methyl EM-A (ME36), EM-B enolether (ME67), N-ethyl, N-methyl EM-A enolether (EM523), ABT-229 and 4′′ deoxy EM-A enolether (KOS1326)] in a Chinese hamster ovary (CHO)-K1 cell line expressing the human motilin receptor (MTLR) and in rabbit duodenal segments. CHO-MTLR cells were preincubated with motilides prior to stimulation with motilin. The negative logarithm of the preincubation concentration reducing the maximal motilin-induced Ca2+ flux to 50% was calculated (pDC50). Internalization was visualized in CHO-K1 cells containing an enhanced green fluorescent protein (EGFP)-tagged MTLR and quantified in binding experiments. The contractile response of repeated stimulations was measured in duodenal segments. In CHO-MTLR cells, the pDC50 was ABT-229 (8.78) > motilin (7.77) > EM-A (4.78), different from their order of potency to induce Ca2+ release (pEC50): motilin (9.39) > ABT-229 (8.46) > EM-A (7.11). In cells with the EGFP-tagged MTLR, ABT-229 decreased membrane fluorescence by 25 ± 2% compared with 16 ± 2% for motilin and 8 ± 2% for EM-A. Binding studies confirmed that EM-A did not induce MTLR internalization (residual binding 96 ± 4% compared with motilin, 31 ± 3% and ABT-229, 21 ± 1%). Comparison of the pDC50 and pEC50 values of the other motilides ME4 (5.90; 8.08), ME67 (6.03; 8.12), ME36 (3.32; 6.62), EM-523 (6.02; 8.22), and KOS1326 (7.32; 8.14) suggested that the strong desensitizing properties of ABT-229 are mostly related to the removal of the 4″-OH of the cladinose sugar. The decline of the contractile response in duodenal segments correlated with the pDC50. The ability to desensitize and internalize the MTLR is not only determined by potency. This may be an important criterion for the development of a clinically useful compound.
Footnotes
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This work was supported by grants from the Flemish Foundation for Scientific Research (FWO G.0144.04) and the Belgian Ministry of Science (GOA 03/11 and IAP P5/20).
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.104.081497.
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ABBREVIATIONS: EM-A, erythromycin-A; CHO, Chinese hamster ovary; MTLR, motilin receptor; EGFP, enhanced green fluorescent protein; ABT-229, N-ethyl, N-methyl 4′′ deoxy EM-B enolether; MTLR-EGFP, EGFP-tagged motilin receptor; PBS, phosphate-buffered saline; BSA, bovine serum albumin; PCR, polymerase chain reaction; ACh, acetylcholine; ME4, EM-A enolether; ME36, N-ethyl, N-methyl EM-A; ME67, EM-B enolether; EM523, N-ethyl, N-methyl EM-A enolether; KOS1326, 4′′ deoxy EM-A enolether.
- Received November 30, 2004.
- Accepted March 9, 2005.
- The American Society for Pharmacology and Experimental Therapeutics
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