The level of bioavailable growth hormone (GH) after the first GH injection predicts the first year's growth response in GH-deficient children

Serum GH levels were measured in 9 prepubertal children with growth hormone (GH) deficiency using an immunofluorometric assay (IFMA) and a Nb₂ cell bioassay, prior to and 2, 4, 6 and 12 hours after the first hGH subcutaneous injection (sc) (0.1 IU/Kg). After this first acute phase, hGH treatment was continued regularly at a dose of 0.1 IU/kg per day at bedtime. The acute pharmacokinetic profile was similar in the patients irrespectively of the assay used: serum GH usually starts to rise after 2 hours, peaks by 4 or 6 hours and drops back to near baseline levels 12 hours after sc GH administration. A great variation in the amplitude of peak levels was observed among the patients. The assays significantly (p < 0.0001) correlate with each other in all subjects. An increase in serum GH values as evaluated by the Nb₂ bioassay was observed in most children after sc GH injection, suggesting that the administered hormone preserves its biological activity. The individual variations in GH bioactivity in our GH-deficient children could be due to different mechanisms of the drug's degradation at the site of injection or in circulation. Pre-treatment height velocity calculated as standard deviation scores (SDS) significantly increased (p < 0.001) from -2.77 ± 0.42 to 1.22 ± 0.87 SDS after a 12-month period of GH treatment in 8 patients who had already completed the first year of therapy, although it varied considerably among subjects. A higher serum GH peak value evaluated by both assays corresponded to a higher growth response during the 1st year, but not in the 2nd year of GH therapy. In conclusion, the degree of GH bioactivity released into circulation after sc GH injection may vary considerably among GHD patients and correlates with the growth response observed during the 1st year of GH therapy.