Carbohydrate-targeted optimization of therapeutic gonadotropins - Carbohydrate Chemistry: Chemical and Biological Approaches - page 466

turn, inhibins suppress FSH. Interestingly, inhibin B was shown to
rise in the early follicular phase and then decreases progressively during
the luteal phase apart from the midcycle preovulatory LH peak. By
contrast, the inhibin A concentration is low in the early follicular
phase, rises at ovulation, and is maximal during the midluteal phase. 34,35
A recent study described that more acidic (sialylated) and complex re-
combinant hFSH (rhFSH) glycoforms showed a potent stimulatory effect
on inhibin B production whereas less acidic (sialylated) and hybrid-type
glycoforms favoured inhibin A production. 36 Moreover, the pregnancy,
implantation rates, metaphase II oocytes and grade 1 embryos were stat-
istically significantly lower in the less-acidic rhFSH compared to combined
sequential acidic hFSH/less-acidic rhFSH, and acidic hFSH. 37 Taken
together, these data indicate that the extent of sialylation and the com-
plexity of oligosaccharides present in FSH are key physiological factors for
oocyte maturity, embryo cleavage and favorable clinical outcome. Nearly
30% less complex glycoforms are found at midcycle than in the early to
mid-follicular, late-follicular and luteal phases (Table 1). 33
Thus, there is a direct relationship between FSH sialylation and
bioactivity. 33 Moreover, FSH is found as bi- and tetraantennary glyco-
forms. Whereas the biantennary isoform exhibits two peaks (day 5 to 7
and midcycle), tetraantennary glycoforms plateaued at a high concen-
tration from day 5 to 15, without a midcycle peak. 38 A decrease in com-
plex and branched glycoforms is also observed during the menopause
compared to the glycoforms secreted during the follicular phase of the
normal menstrual cycle. 33,39
3.2.2 Natural LH. Pituitary stock of LH displays predominantly
mono- and biantennary glycans. 30,40 One of the striking differences
between pituitary FSH and LH is that sialylated glycoforms are less
represented in LH to the benefit of GalNAc-sulfate structures (58% of
sialylated vs. 49% of sulfated glycoforms in hLH and 88% of sialylated
vs. 7% of sulfated glycoforms in hFSH). 30 The gonadotroph cells of the
pituitary have the remarkable ability to segregate LH and FSH into
distinct secretory granules. It was suggested that the glycans present on
LH and FSH may act as recognition signal for sorting out and secreting
these products independently. The presence of predominantly sialic acid
residues in FSH and sulfate residues in LH may very well help targeting
each hormone to separate secretory granules. 41 No significant difference
is observed in the proportion of LH with pIW6.55 between midcycle
(53%) and early to mid-follicular (36%), late-follicular (43%) and luteal
phases (32%) although all were higher than that found in the meno-
pausal hormone (13%). Moreover, less complex LH glycoforms are
found at midcycle (13%) than in the early to mid-follicular (18%), late
follicular (20%) and luteal phases (24%). Menopausal LH is less complex
(Table 2). 33
3.2.3 Natural hCG. hCG is a particularly complex glycoprotein in
that 25-30% of its molecular weight is composed of carbohydrates up to
35-41% in hyperglycosylated hCG.