Effect of injection site on the pharmacokinetics and pharmacodynamics of subcutaneously administered luteinizing hormone releasing hormone.

Serum concentrations of LHRH and the subsequent LH responses were compared following s.c. injections of 20 micrograms LHRH into the upper arm and the lower abdominal wall, in 9 hypogonadal women responsive to pulsatile LHRH therapy. Tests were carried out at the two sites in random order. Peak LHRH concentrations were reached by 5 min after upper arm and by 20 min after lower abdominal wall injections, the maximum concentrations being significantly greater following injections into the former. There was no increase in LH until 10 min and then maximum concentrations were reached at 30 min following injection into both sites. There was no significant correlation between the LHRH increments and the LH response but there was a negative correlation between the Ponderal Index of the patients and the LHRH increments following injections into the lower abdominal wall only. There was no significant overall difference between the LH increments related to the site of injection, but the order of injections affected the responses. When upper arm injections were given first the LH responses were significantly greater, but when lower abdominal wall was injected first the subsequent responses to upper arm injections were impaired. A possible reason for this is that the absorption from the lower abdominal wall was delayed so prolonging the exposure of the pituitary gonadotrophs to LHRH, resulting in pituitary desensitization at the time of the second test.