Flip-flop phenomenon

Lin PI, Vance JM, Pericak-Vance MA, Martin ER. 2007. No gene is an island The flip-flop phenomenon. Am J Hum Genet 80 531-538. 

Tissue distribution studies were also done in rats after nasal administration. As seen in Figure 14 and Table 2, it was found that the elimination half-life of the unmodified sCT was 199 min, whereas the SP-mPEG2000-modified sCT showed an increased terminal elimination with a half-life of 923 min. It was also found that the SP-mPEG2000-modified sCT took a significantly longer time to reach its maximum concentration, 520 min, as compared with the 77 min for the unmodified sCT, and the AUC was found to be 20,638 pg/min/mL, which is much higher than the 3650 pg/ min/mL for the unmodified sCT. The authors reported that the increase in the terminal half-life observed could be due to a flip-flop phenomenon. Also, when the tissue distribution of the formulation was examined 12 h after administration, the highest radioactivity was found in the liver. The details of the biodistribution studies are as shown in Table 3. 

When absorption takes place rapidly, which is usual for conventional dosage forms, fca > kd, but when absorption takes place slowly and ka < k(j the flip-flop phenomenon occurs, whereby the rate of absorption controls the rate of elimination of the drug. This situation applies not only to sustained-release oral dosage forms administered to dogs, but also to phenylbutazone and meclofe-namic acid in horses, salicylate administered as an aspirin bolus to cattle and oral suspensions of benzimidazole anthelmintics in ruminant species. 

Flip-flop spin diffusion is a phenomenon specific for NMR. It does not matter with other techniques. Spin diffusion based on Zeeman energy conserving flip-flop transitions of dipolar coupled spins, that is interchange of spin-... 

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