The oral 17 alkylated testosterone derivates, methyltestosterone and fluoxymes-terone, are not recommended for long-term androgen replacement because of potential adverse effects on liver function and lipid levels. Testosterone undecanoate is a testosterone ester with a long fatty acid side chain. When administered by mouth, testosterone is absorbed by lymphatics and must be administered after ingestion of food. Serum testosterone levels rose to peak levels 4 to 5 h after administration and remained in low normal range 8 to 12 h after oral administration (80,81). When testosterone undecanoate is administered in the fasting state, serum testosterone levels remained low. Testosterone undecanoate has long-term safety data (82). There are marked inter-subject as well as intrasubject variations in the peak serum testosterone levels achieved after oral administration. The usual dose is testosterone undecanoate 80 mg bid or 40 mg tid. This oral preparation is not available in the United States but is available in Canada, Mexico, Europe, Asia, and Australia.
Several new oral preparations are being developed for the U.S. market but have not been approved by the Food and Drug Administration (FDA). Buccal administration of testosterone (83) and sublingual testosterone cyclodextrin (84) result in high peaks and troughs lasting for approx 2 h. The PKs are represented as the sharp peaks and troughs in Fig. 2. Although sublingual testosterone improved sexual function, mood, and muscle mass, this preparation is not available for androgen replacement (16). A more recent development is a buccal tablet, which, after application to the gum, gels and results in steady serum testosterone levels. The buccal bioadhesive tablet has to be administered twice per day, does not appear to cause gum irritation, and delivers steady serum testosterone levels (85).
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