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This technology is most often applied to valuable animals (e.g. prize-winning horses, or high milk-yield dairy cattle) in order to boost their effective reproductive capacity several-fold. Each of the offspring will inherit its genetic complement from the biological mother (and father), irrespective of which recipient animal carries it to term.
Gonadotrophins are usually utilized to induce a superovulatory response, and include porcine FSH (p-FSH), porcine LH (p-LH) and PMSG. P-FSH is extracted from the pituitary glands of slaughterhouse pigs. The crude pituitary extract is usually subject to a precipitation step, using either ethanol or salts. The FSH-containing precipitate is normally subjected to at least one subsequent chromatographic step. The final product often contains some LH as well as low levels of additional pituitary-derived proteins.
p-LH is obtained, again, by its partial purification from the pituitary glands of slaughterhouse pigs. In both cases, a porcine source is utilized, as the target recipients are almost always cattle or horses. The use of a product derived from a species other than the intended recipient species is encouraged, as it helps minimize the danger of accidental transmission of disease via infected source material (many pathogens are species-specific).
Most superovulatory regimes utilizing p-FSH entails its administration to the recipient animal twice daily for 4-5 days. Regular injections are required, due to the relatively short halflife of FSH in serum; s.c. administration helps prolong the duration of effectiveness of each injection. The 4 or 5 days of treatment with FSH is followed by a single dose of LH, promoting final follicular maturation and ovulation.
The causes of variability of superovulatory responses are complex, and not fully understood. The general health of the animal, as well as its characteristic reproductive physiology, is important. The exact composition of the gonadotrophin preparations administered and the exact administration protocol also influences the outcome. The variability of FSH:LH ratios in many p-FSH preparations can affect the results obtained, with the most consistent superovulatory responses being observed when FSH preparations exhibiting low LH activity are used. The availability of recombinant FSH and LH will overcome these difficulties at least.
An alternative superovulatory regime entails administration of PMSG which, as described earlier, exhibits both FSH and LH activity. The major rationale for utilizing PMSG is its relatively long circulatory half-life. In cattle, clearance of PMSG may take up to 5 days. The
HORMONES OF THERAPEUTIC INTEREST 345
Table 8.13. GnRH and some of its analogues, along with their amino acid sequence
Analogue Amino acid sequence
Buserelin acetate C2H5-NH-Pro-Arg-Leu-Ser-Tyr-Ser-Trp-His-5oxo pro
Goserelin acetate NH2CO(NH)2-Pro-Arg-Leu-Ser-Tyr-Ser-Trp-His-5oxo pro
Leuprorelin acetate C2H5-NH-Pro-Arg-Leu-D-Leu-Tyr-Ser-Trp-His-5oxo pro
Nafarelin acetate NH2-Gly-Pro-Arg-Leu-D-Ala-Tyr-Ser-Try-His-5oxo pro
GnRH NH2-Gly-Pro-Arg-Leu-Gly-Tyr-Ser-Trp-His-5oxo pro
slow clearance rate appears to be due to the molecule’s high content of N-acetyl-neuramic acid. This extended serum half-life means that a single dose of PMSG is sufficient to induce a superovulatory response. Paradoxically, however, its extended half-life limits its use in practice. Post-ovulatory stimulation of follicular growth can occur, resulting in the recovery of a reduced number of viable embryos. Attempts to negate this biological effect have centred around administration of anti-PMSG antibodies several days after PMSG administration. However, this gonadotrophin is still not widely used.
Gonadotrophin releasing hormone (GnRH)
GnRH is also used (in both human and veterinary medicine) to improve conception rates by enhancing basal hypothalamic-pituitary function. The preparations utilized clinically are manufactured by direct chemical synthesis and are usually administered by s.c. injection or, sometimes, by i.v. injection. Occasionally it has also been administered intranasally. Frequent injection is often required, particularly if administration is via the i.v. route, as the peptide’s plasma half-life is of the order of a few minutes (it is hydrolysed in the plasma and excreted in the urine).
Direct chemical synthesis facilitates the manufacture of GnRH analogues of altered amino acid sequence. Several such analogues display useful clinical properties (such as extended halflife). Analogues used medically (Table 8.13) have activities similar to those of GnRH, and are used not only in the treatment of reproductive complications but also for the treatment of malignant neoplasms of the prostate and breast. Gonadal steroids can promote growth of many such tumours and a single s.c. injection of some analogues can suppress testosterone and oestradiol synthesis for up to 4 weeks. Interestingly, GnRH analogues have been reported to relieve pre-menstrual tension.