Factors determining the intrinsic lymphatic partition rate of epitiostanol and mepitiostane.

Substitution of the steroid epitiostanol (EP) at position 17 with methoxycyclopentane yields the extremely liophilic mepitiostane (MP) with preferential partitioning into the lymph. Most of the MP in the lymph was associated with the core lipids of chylomicrons and very low-density lipoproteins (VLDL), as was also the case for EP. However, the dialysis velocity of EP and MP from lymph to plasma differed greatly; EP, but not MP, was transferred from the lymph to the plasma. This difference was attributed to differences in their unbound fraction in the lymph. Lymphatic transfer and the logP value of several tested steroids correlated well. Therefore, the oral EP prodrug, MP, partitioned into the lymph because of its superlipophilicity and resultant retention in the core lipids of chylomicrons and VLDL.