Long-Acting Growth Hormone for Replacement Therapy

Abstract

GH replacement therapy has shown important beneficial effects on body composition, bone metabolism, lipid metabolism, and quality of life in adults with GH deficiency with a good safety profile (1, 2). Current management is based on daily sc injections before bedtime that are often experienced as cumbersome by the patient and may therefore affect adherence to treatment. Sustainedrelease GH formulations or long-acting GH for less frequent injections have therefore been developed. The paper by Biller et al. (3) published in this issue of the JCEM reports the results from a large 6-month study of adults with GH deficiency demonstrating efficacy and safety of a sustained-release formulation of GH with once weekly sc administration. The study concluded that the treatment was well tolerated and all clinical beneficial end-points were met, such as reduction in fat mass. GH is a powerful metabolic hormone with lipolytic actions, protein anabolic effects, and insulin antagonistic effects (4). The plasma pattern of GH, including pulse frequency, basal levels, and total integrated levels is influenced by several factors, including gender, age, and adiposity (5). Therefore, pattern of GH delivery and GH exposure may have an impact on the effects obtained by treatment. The importance of the mode of GH administration is supported by a number of animal studies demonstrating that a changed pattern of GH exposure has different effects on growth and metabolism (6). The GH secretion is suppressed by food and augmented by fasting. It is therefore a fasting hormone mobilizing energy from fat stores by lipolysis, inducing gluconeogenesis in the liver, and preserving protein mass during fasting through its protein anabolic action, i.e. in many aspects the reverse action to insulin in the postprandial state (7). Moreover, the gene expression and activity of some important regulatory enzymes in the liver are dependent on the pattern of GH exposure (8, 9). The clinical importance for safety and efficacy of not delivering GH in a physiological pulsatile manner is therefore something to consider in the pharmaceutical development of GH products for treatment. Current practice of GH treatment both in children and in adults is to administer GH as a bedtime sc injection. The basis for this practice comes from early observations that daily sc injections gave a better growth response in GHdeficient children than three injections per week (10). Moreover, the closest similarity to a normal diurnal metabolite pattern was achieved with evening sc injections of GH to GH-deficient adults (11). The diurnal plasma GH profile following daily sc evening injections is, however, an intermediate between a pulsatile and a continuous GH exposure time profile (12) and is therefore not physiological. The bedtime administration of GH means that most of the GH action occurs during the night in the fasting state. This is, however, the regime that has proved its safety and efficacy of modern GH therapy in children and in adults (1). Having a more continuous exposure, such as during treatment with long-acting GH, means that GH exerts its action both in fasting and in the fed state during the day, meaning that the normal interaction and balance with insulin is further set aside. The clinical importance of this is not clear. Whether the same efficacy and safety can be obtained by moving the GH exposure toward a more continuous pattern has been studied mainly in adults. In one small pediatric study, however, GH delivered either as daily sc injections or as a continuous GH administration using a sc pump demonstrated the same growth rate by the two treatment modes, but the serum IGF-I response was higher in the group receiving continuous GH (13). Studies in adults comparing the same dose of GH administered as a daily sc injection or as a continuous infusion have, in agreement with the pediatric experience, shown that the IGF-I and ISSN Print 0021-972X ISSN Online 1945-7197 Printed in U.S.A. Copyright © 2011 by The Endocrine Society doi: 10.1210/jc.2011-0689 Received April 20, 2011. Accepted April 22, 2011.