Dutogliptin, a selective DPP4 inhibitor, improves glycaemic control in patients with type 2 diabetes: a 12-week, double-blind, randomized, placebo-controlled, multicentre trial.

AIM: To determine efficacy and tolerability of dutogliptin, a dipeptidyl peptidase 4 (DPP4) inhibitor, in patients with type 2 diabetes mellitus. METHODS: This was a 12-week, multicentre, randomized, double-blind, placebo-controlled trial in 423 patients with type 2 diabetes with suboptimal metabolic control. Following a 2-week single-blind placebo run-in, patients aged 18-75 years with a body mass index of 25-48 kg/m(2) and baseline HbA1c of 7.3-11.0% were randomized 2:2:1 to receive once-daily oral therapy with either dutogliptin (400 or 200 mg) or placebo on a background medication of either metformin alone, a thiazolidinedione (TZD) alone or a combination of metformin plus a TZD. RESULTS: Average HbA1c at baseline was 8.4%. Administration of dutogliptin 400 and 200 mg for 12 weeks decreased HbA1c by -0.52% (p < 0.001) and -0.35% (p = 0.006), respectively (placebo-corrected values), with absolute changes in HbA1c for the 400 mg, 200 mg and placebo groups of -0.82, -0.64 and -0.3%, respectively. The proportion of patients achieving an HbA1c < 7% was 27, 21 and 12% at dutogliptin doses of 400 and 200 mg or placebo, respectively (p = 0.008 for comparison of 400 mg vs. placebo). Fasting plasma glucose (FPG) levels were significantly reduced in both active treatment groups compared to placebo: the placebo-corrected difference was -1.00 mmol/l (p < 0.001) for the 400 mg group and -0.88 mmol/l (p = 0.003) for the 200 mg group. Dutogliptin caused significantly greater reductions in postprandial glucose AUC (0-2h) in both the 400 and 200 mg groups (placebo corrected values -2.58 mmol/l/h, p < 0.001 and -1.63 mmol/l/h, p = 0.032, respectively). In general, patients tolerated the study drug well. There were minor, not clinically meaningful differences in adverse events (AEs) between dutogliptin-treated patients and placebo controls, and 60% of all reported AEs were mild. Vital signs and body weight were stable, and routine safety laboratory parameters did not change compared with placebo. Trough ex vivo DPP4 inhibition at the end of the 12-week treatment period was 80 and 70%, at the 400 and 200 mg doses of dutogliptin, respectively. CONCLUSIONS: Dutogliptin treatment for 12 weeks improved glycaemic control in patients with type 2 diabetes who were on a background medication of metformin, a TZD or metformin plus a TZD. Tolerability was favourable for both doses tested. The 400 mg dose of dutogliptin resulted in larger changes of HbA1c and FPG and more subjects reached an HbA1c target of < 7% than the 200 mg dose.

The dipeptidyl peptidase-4 inhibitor PHX1149 improves blood glucose control in patients with type 2 diabetes mellitus.

AIM: To determine the efficacy and tolerability of PHX1149, a novel dipeptidyl peptidase-4 (DPP4) inhibitor, in patients with type 2 diabetes. METHODS: This is a multicentre, randomized, double-blind, placebo-controlled, 4-week study in patients with type 2 diabetes with suboptimal metabolic control. Patients with a baseline haemoglobin A(1c) (HbA(1c)) of 7.3 to 11.0% were randomized 1 : 1 : 1 : 1 to receive once-daily oral therapy with either PHX1149 (100, 200 or 400 mg) or placebo; patients were on a constant background therapy of either metformin alone or metformin plus a glitazone. RESULTS: Treatment with 100, 200 or 400 mg of PHX1149 significantly decreased postprandial glucose area under the curve AUC(0-2 h) by approximately 20% (+0.11 +/- 0.50, -2.08 +/- 0.51, -1.73 +/- 0.49 and -1.88 +/- 0.48 mmol/l x h, respectively, for placebo and 100, 200 and 400 mg (p = 0.002, 0.008 and 0.004 vs. placebo). Postprandial AUC(0-2 h) of intact glucagon-like peptide-1, the principal mediator of the biological effects of DPP4 inhibitors, was increased by 3.90 +/- 2.83, 11.63 +/- 2.86, 16.42 +/- 2.72 and 15.75 +/- 2.71 pmol/l x h, respectively, for placebo and 100, 200 and 400 mg (p = 0.053, 0.001 and 0.002 vs. placebo). Mean HbA(1c) was lower in all dose groups; the placebo-corrected change in the groups receiving 400 mg PHX1149 was -0.28% (p = 0.02). DPP4 inhibition on day 28 was 53, 73 and 78% at 24 h postdose in the groups receiving 100, 200 and 400 mg PHX1149, respectively. There were no differences in adverse events between PHX1149-treated and placebo subjects. CONCLUSIONS: Addition of the DPP4 inhibitor PHX1149 to a stable regimen of metformin or metformin plus a glitazone in patients with type 2 diabetes was well tolerated and improved blood glucose control.

Insulin-like growth factor-1 lowers protein oxidation in patients with thermal injury.

OBJECTIVE: The effect of insulin-like growth factor-1 (IGF-1) on energy expenditure and protein and glucose metabolism in a group of patients with thermal injury was determined. SUMMARY BACKGROUND DATA: Accelerated protein catabolism is a constant feature of the hypermetabolic response to thermal injury. Insulin-like growth factor-1 has been reported to minimize protein catabolism and normalize energy expenditure in animal models of thermal injury. METHODS: To determine the efficacy of IGF-1 in human burn patients, resting energy expenditure (metabolic cart), whole body protein kinetics (N15 Lysine), and glucose disposal (glucose tolerance test) were assessed in eight burn patients before and after a 3-day infusion of IGF-1 (20 micrograms/kg/hr). All patients were fluid-resuscitated uneventfully and were without obvious infection at the time of study. Enteral nutrition was administered at a constant rate before and during the IGF-1 infusion. RESULTS: Resting energy expenditure was not altered by IGF-1 (40.3 +/- 2.2 vs. 39.1 +/- 2.3 kcal/kg/day). However, glucose uptake was promoted, and protein oxidation decreased significantly (0.118 +/- 0.029 vs. 0.087 +/- 0.021 g/kg/d, p < 0.05) by IGF-1. In addition, insulin secretion, in response to a glucose challenge, was blunted. CONCLUSIONS: Insulin-like growth factor-1 therapy has a beneficial effect in preserving lean body mass during severe stress conditions by minimizing the flux of amino acids toward oxidation.

Clinical uses of insulin-like growth factor I.

Insulin-like growth factor I (IGF-I) has acute insulin-like metabolic effects and long-term anabolic actions. The therapeutic potential of recombinant human IGF-I treatment is being investigated in various growth hormone-resistant and insulin-resistant disorders. Recent studies have shown that IGF-I may substitute for growth hormone in promoting linear growth in children with growth hormone insensitivity. The anabolic, protein-sparing action of IGF-I is being evaluated as a potential therapy for adults with catabolic diseases. Patients with insulin-dependent diabetes mellitus have reduced endogenous IGF-I production, and studies are in progress to determine whether treatment with IGF-I in addition to insulin may improve their metabolic/anabolic status. Insulin-like growth factor I treatment may reduce glucose and triglyceride levels in adults with non-insulin-dependent diabetes mellitus and in some patients with extreme insulin resistance. Further studies are needed to evaluate the efficacy and safety of IGF-I treatment in these and other conditions and to provide a better understanding of this hormone's normal physiologic role(s) and complex relations with growth hormone and insulin.

Adverse effects of recombinant human insulin-like growth factor I in obese insulin-resistant type II diabetic patients.

Data from studies in diabetic rodents and evidence from clinical situations of severe resistance to insulin suggest that insulin-like growth factor I (IGF-I) is able to at least partly overcome insulin resistance. To assess the efficacy of recombinant human IGF-I in subjects with the most common form of insulin resistance, e.g., obese, type II diabetic patients, we administered recombinant human IGF-I (rhIGF) in doses of 120 and 160 micrograms/kg twice daily for 4-52 days to seven such individuals who had been treated previously with high doses of insulin (> 0.7 U.kg-1 x day-1). Four patients exhibited comparable or enhanced, whereas three had diminished, blood glucose control on rhIGF-I relative to that while on twice daily NPH insulin during the six-week control period. The occurrence of adverse effects in all patients compelled us to discontinue rhIGF-I administration before completing the 8-week treatment period. These adverse effects included edema primarily on the face and hands, mild weight gain, occasional dyspnea, bilateral jaw tenderness, arthralgias and myalgias, fatigue, tachycardia, flushing, orthostatic hypotension, and local burning at the injection site. We conclude that the frequency and severity of side effects associated with administering high-dose subcutaneous rhIGF-I to obese insulin-resistant diabetic patients make it an unacceptable therapeutic agent for these patients despite its ability to produce reasonable blood glucose control in approximately 50% of them.

Short-term effects of recombinant human insulin-like growth factor I on metabolic control of patients with type II diabetes mellitus.

Recombinant human insulin-like growth factor I (rhIGF-I) lowers blood glucose, serum insulin, C-peptide, and lipid levels in healthy and diabetic animals and humans. We hypothesized that rhIGF-I might control blood glucose levels and concomitantly reduce pancreatic insulin secretion in patients with type II diabetes. If true, rhIGF-I might serve as a therapeutic agent that could mitigate some of the detrimental effects of hyperinsulinemia secondary to insulin resistance in these patients. In this study, we treated 12 patients with type II diabetes mellitus twice daily for 5 days with sc rhIGF-I in doses of 90, 120, or 160 micrograms/kg body weight. Metabolic parameters in the fasting and postprandial states were assessed during a 3-day baseline period, the rhIGF-I treatment period, and a 3-day follow-up period, respectively. Administration of rhIGF-I significantly reduced mean (+/- SD) concentrations of fasting blood glucose (12.3 +/- 4.5 to 9.1 +/- 2.6 mmol/L), serum insulin (98 +/- 52 to 56 +/- 27 pmol/L), and C-peptide (993 +/- 298 to 728 +/- 232 pmol/L). It also decreased postprandial (area under the curve) blood glucose (32.5 +/- 12.7 to 23.9 +/- 8.1 mmol/L.h), serum insulin (1102 +/- 707 to 467 +/- 332 pmol/L.h), and C-peptide (5958 +/- 2747 to 3442 +/- 1523 pmol/L.h). The administration of rhIGF-I was also associated with a small but significant reduction in serum triglycerides (6.76 +/- 3.45 to 5.32 +/- 2.59 mmol/L) and total cholesterol (6.13 +/- 1.25 to 5.66 +/- 1.20 mmol/L), 24-h creatinine clearance increased significantly (85 +/- 30 to 133 +/- 51 mL/min), and microalbuminuria was unchanged. Although rhIGF-I was reasonably well tolerated, side effects included low-grade edema, mild and mainly asymptomatic orthostatic hypotension, and bilateral temporomandibular tenderness. We conclude that short-term treatment of type II diabetic patients with rhIGF-I favorably affects metabolic control and enhances kidney function. An assessment of the risk/benefit ratio of rhIGF-I administration to this group of patients awaits extended experiments.

Hypoglycemic and insulin response to a continuous intravenous infusion of CGP-35126 recombinant human insulin-like growth factor-I (rhIGF-I) in healthy males.

Recombinant human insulin-like growth factor-I (rhIGF-I) produced by expression in a yeast vector was evaluated in seven normal men to determine effects on plasma glucose and insulin levels. Each subject received an initial intravenous infusion of normal saline and on the following day, rhIGF-I at a rate of 21.4 micrograms/kg/hour. Each infusion lasted for 7 hours. The subjects' fasting baseline glucose and insulin levels were not statistically different (P > .05) from their pre-dose levels. Compared with the saline (control) infusion, serum glucose levels were statistically lower (P < .05) 2 hours into the rhIGF-I infusion. These lower glucose levels were maintained until the subjects consumed a standard lunch (4 hours into the infusion). Insulin levels demonstrated a similar response to rhIGF-I, but decreases in insulin levels occurred after the rhIGF-I hypoglycemic effect. This observation suggests that suppression of insulin levels may be due to secondary hypoglycemia rather than to a direct rhIGF-I effect. This study demonstrated the desired rhIGF-I effect of lowering subjects' glucose levels without clinically significant hypoglycemia. This finding suggests that rhIGF-I may have potential clinical utility in hyperglycemic states.

Effects of insulin-like growth factor I on renal function in normal men.

Acute and chronic studies in rats have shown that administration of human recombinant insulin-like growth factor I (rhIGF-I) lowers renal vascular resistance and increases RPF, GFR and proximal tubular phosphate absorption. In the present study we examined the effects of subcutaneous injections of rhIGF-I on glomerular and tubular function in eight normal men. Individuals were studied for 5.5 consecutive days in a clinical research center while they ate a constant diet. Four subjects were studied in a non-volume expanded state (Group 1) and four individuals were evaluated during a saline load. From the second to the fourth day, subjects received subcutaneous injections of rhIGF-I, 60 micrograms/kg, at 0800, 1400 and 2000 hours. After commencing the rhIGF-I injections, serum IGF-I levels rose quickly and remained at about three to four times that of baseline throughout the period of rhIGF-I injections. In both the normal and the saline loaded subjects, renal vascular resistance decreased and RPF and GFR (PAH and inulin clearances) rose quickly and were clearly altered within six hours after starting the rhIGF-I injections. RPF had increased by 32 +/- 3% and 33 +/- 2% (grand mean +/- SEM) in the normal and the saline loaded subjects, and GFR rose by 22 +/- 3% and 36 +/- 4% in the two groups. In both groups the absolute and the fractional excretion of phosphate decreased markedly during rhIGF-I treatment, but the absolute and fractional excretion of calcium did not change. The urinary fractional and absolute excretion of albumin and IgG also increased, although slightly, with rhIGF-I injections. There was no consistent effect of IGF-I on tubular sodium handling. These findings demonstrate that in normal men subcutaneous injections of rhIGF-I greatly increase RPF, GFR, and tubular phosphorus reabsorption and enhances microproteinuria.

Regulation of binding proteins for insulin-like growth factors (IGF) in humans. Increased expression of IGF binding protein 2 during IGF I treatment of healthy adults and in patients with extrapancreatic tumor hypoglycemia.

UNLABELLED: Insulin-like growth factors (IGFs) in blood form two complexes with specific binding proteins (BPs): a large, growth hormone (GH)-dependent complex with restricted capillary permeability, and a smaller complex, inversely related to GH, with high turnover of its IGF pool and free capillary permeability. The distribution of BPs and of IGFs I and II between these complexes was studied in sera from healthy adults treated with IGF I or/and GH and from patients with extrapancreatic tumor hypoglycemia. Like GH, IGF I administration raises IGF I and two glycosylation variants of IGFBP-3 in the large complex, but unlike GH drastically reduces IGF II. During IGF I infusion, IGFBP-3 appears in the small complex whose IGFBP-2 and IGF I increase three- to fivefold and fivefold, respectively. GH treatment, associated with elevated insulin levels, suppresses IGFBP-2 and inhibits its increase owing to infused IGF I. The small complex of tumor sera contains increased amounts of IGFBP-2 and -3, and two- to threefold elevated IGF II. CONCLUSIONS: low GH and/or insulin during IGF I infusion and in extrapancreatic tumor hypoglycemia enhance expression of IGFBP-2 and favor partition of IGFBP-3 into the small complex. Free capillary passage and high turnover of its increased IGF I or II pools may contribute to compensate for suppressed insulin secretion during IGF I infusion or to development of tumor hypoglycemia.

Therapeutic potential of insulinlike growth factor i.

Human recombinant insulinlike growth factor I is a promising therapeutic agent for diseases characterized by relative insulin resistance, e.g., diabetes mellitus, obesity, and hypertriglyceridemia, since it suppresses growth hormone, insulin, C-peptide, and triglyceride levels and lowers the total cholesterol to high-density lipoprotein-cholesterol ratio. Moreover, insulinlike growth factor administration increases kidney function in healthy subjects (glomerular filtration rate, renal plasma flow) and may prove useful in the treatment of degenerative disorders of cartilage and bone (arthrosis, osteoporosis) as well as in catabolic states.

Effects of insulin-like growth factor I in man.

A 6-day period of subcutaneous infusion with recombinant human insulin-like growth factor I in three healthy male volunteers resulted in an increase in the ratio of insulin to C-peptide levels and significant decreases in triglyceride levels and the ratio of total to high density lipoprotein-cholesterol in serum. Increased renal plasma flow and glomerular filtration rates were also observed.

[Insulin-like growth factors (IGF I and IGF II) and diabetes]. / Insulinähnliche Wachstumsfaktoren (IGF I und IGF II) und Diabetes.

IGF I and IGF II are insulin-like peptide hormones circulating in blood bound to specific carrier proteins. IGF I mediates many actions of growth hormone--hence its designation as somatomedin C--and it stimulates cell replication, cell differentiation and the synthesis of cellular products (matrix proteins, etc.). Recent investigations in normal human subjects show that pharmacological doses of IGF I increase renal function and have profound metabolic effects that might prove to be useful in the treatment of conditions characterized by relative insulin resistance, such as type 2 diabetes, obesity and hyperlipidemia.

Insulin-like growth factors I and II in healthy man. Estimations of half-lives and production rates.

IGF-I and -II share specific serum carrier proteins which elute on neutral Sephadex G-200 gel permeation chromatography at apparent molecular masses of 50 and 200 kD. The half-lives of free and carrier protein-bound 125I-IGF-I and -II were determined after bolus injections of the tracers into two normal adults. Labelled IGF-I and -II migrated first with the 50-kD and later with the 200-kD complex. In these complexes their apparent half-lives were 20-30 min and 12-15 h, respectively. The apparent half-life a free 125I-IGF-I and -II was 10-12 min. In a second set of experiments, recombinant human insulin-like growth factor I was infused during 6 days in two healthy adults at a dose of 20 micrograms.kg-1.h-1 (corresponding to around 30 mg/day). Serum obtained before and during the infusion was subjected to neutral Sephadex G-200 gel permeation chromatography and fractions were pooled according to the apparent molecular masses at which the carrier protein complexes elute. IGF-I and -II in these pools were determined by RIA. Before the IGF-I infusion, 92 and 272 micrograms/l of IGF-I and -II were found in the 200-kD complex, 45 and 91 micrograms/l in the 50-kD complex, and 15 and 5 micrograms/l were present in the free form. Corresponding figures during the IGF-I infusion were 389 and 18 micrograms/l for the 200-Kd complex, 201 and 54 micrograms/l for the 50-kD complex, and 80 and less than 1 microgram/l for free IGF-I and -II. Using the half-lives of the tracer studies and the levels of the different molecular weight forms of IGF in serum, the production rates for IGF-I and -II were calculated to be 10 mg and 13 mg per day.

Small stature and insulin-like growth factors: prolonged treatment of mini-poodles with recombinant human insulin-like growth factor I.

The short stature of mini-poodles is associated with low serum levels of IGF-I. Standard poodles are taller and have considerably higher serum levels of IGF-I. Low IGF-I serum levels may be a symptom or the cause of small stature. We, therefore, undertook a study in which serum IGF-I levels of mini-poodles were elevated over a prolonged period of time by a constant infusion of rhIGF-I and the growth rate of the mini-poodles was followed. We infused four mini-poodles from day 91 to day 221 of age with 6 mg/day of recombinant human insulin-like growth factor I (rhIGF-I). Serum levels of IGF-I rose from about 160 to about 500 micrograms/l. Blood glucose remained within normal limits. Stimulation tests with clonidine and with GHRH revealed suppression of endogenous GH secretion during the IGF-I infusion. Serum levels of IGF-II and of creatinine were lower in the IGF-I-infused animals. Radial length and body weight did not increase to a greater extent in the IGF-I infused dogs than in controls. However, 'adapted body mass index' (aBMI = gram body weight/(mm radial length)2) decreased in each of the IGF-I infused animals, whereas it increased in each of the control dogs (p less than 0.05). We conclude that long-term infusion of IGF-I does not stimulate growth in young mini-poodles, but may change body composition.

Insulin-like growth factor I regulates type I procollagen messenger ribonucleic acid steady state levels in bone of rats.

RNA extracted from parietal bones of rats was analyzed for the abundance of type I procollagen mRNA by a dot hybridization assay using specific cDNA probes. Hypophysectomized rats, deficient in IGF I, have 4.1-fold lower steady state levels of mRNA encoding the alpha 1 chain of type I collagen when compared to normal rats of the same weight (120 g). Administration of recombinant human insulin-like growth factor I (IGF I) for 6 days restores bone collagen mRNA levels to normal, and a single sc injection of 100 micrograms IGF I results in a 1.9-fold increase of both alpha 1(I) and alpha 2 (I)chain mRNAs after 8 h. Studies with calvaria cells in primary culture and with a bone-derived cell line, PyMS, yielded similar results: IGF I treatment of these osteoblast-like cells for 24 h increased steady state levels of type I procollagen mRNAs. Thus, IGF I appears to contribute to osteoblast mRNA expression for both chains of type I collagen.

Insulin-like growth factor I increase glomerular filtration rate and renal plasma flow in man.

Recombinant IGF-I was infused seal at a dose of 20 micrograms-kg 1-h 1 to 2 healthy subjects during a total of 79 h. Serum levels of IGF-I rose from 93 and 177 to 502 and 616 micrograms/l, respectively. Fasting blood glucose remained normal. During the infusion glomerular filtration rate increased by 31%, in subject No. 1 and by 32% in subject No.2. Concomitantly, renal plasma flow increased by 26% and 22%, respectively. Proximal and distal tubular reabsorption of fluid and sodium as determined by lithium clearance was elevated to a similar extent. When determined again one week after the end of the IGF-I infusion, all parameters of renal function had returned to baseline. Sodium excretion, body weight and blood pressure did not change. We conclude that IGF-I infused at pharmacological doses has marked effects on kidney function. Future studies will necessary to define the clinical potential of recombinant IGF-I in the treatment of diseases characterized by impaired renal perfusion and filtration.

Recombinant human insulin-like growth factor I induces its own specific carrier protein in hypophysectomized and diabetic rats.

The physiology of the specific serum binding proteins which constitute the main storage pool for insulin-like growth factors (IGFs) in mammals is still incompletely understood. We have, therefore, investigated the regulation of these proteins in (i) hypophysectomized (hypox) rats infused with recombinant human growth hormone (rhGH) or recombinant human IGF 1 (rhIGF I) and (ii) streptozotocin-diabetic rats infused with insulin or rhIGF I. The main carrier protein, a GH-dependent complex of apparent molecular mass 200 kDa, contains N-glycosylated IGF-binding subunits (42, 45, and 49 kDa) that differ in their glycosyl but not in their protein moiety. These subunits are lacking in hypox and diabetic rats. They are induced by GH and insulin, respectively, and appear in the 200-kDa complex. Infusion of rhIGF I induces the subunits in both states; however, only in diabetic, not in hypox, rats do they form the 200-kDa complex. Glycosylated carrier protein subunits do not appear before 8 hr of rhIGF I infusion. During that period, hypox rats may become severely hypoglycemic. After 16 hr, glycosylated subunits are clearly induced, and blood sugar values are normal. We conclude: (i) The N-glycosylated subunits of the 200-kDa complex reflect the IGF I status. (ii) IGF I may mediate the induction of these subunits by GH. (iii) Significant association to the 200-kDa complex occurs only in the presence of GH. It is likely that GH, but not IGF I, induces a component, which itself does not bind IGF, but associates with the glycosylated IGF-binding subunits. (iv) The glycosylated subunits protect against IGF-induced hypoglycemia and may be involved in tissue-specific targeting of IGFs.

Effects of recombinant insulin-like growth factor I on insulin secretion and renal function in normal human subjects.

Insulin-like growth factor I (IGF-I) is an important mediator of growth hormone (GH) action and it appeared tempting to evaluate possible clinical applications. Recombinant IGF-I was infused s.c. at a dose of 20 micrograms/kg of body weight per hour during 6 days in two healthy adult subjects. Blood glucose and fasting insulin levels remained within normal limits and IGF-II levels were suppressed. In contrast to insulin, fasting C peptide levels were decreased. GH secretion was also suppressed by IGF-I. Our preliminary data allow us to distinguish between the effects of GH per se and those of IGF-I: GH causes hyperinsulinism, whereas IGF-I leads to decreased insulin secretion. Glomerular filtration rate, as estimated by creatinine clearance, increased to 130% of preinfusion values during the IGF-I infusion. Total creatinine and urea excretion remained unchanged. We conclude that IGF-I influences kidney function and, in contrast to GH, exerts an insulin-sparing effect. It may be speculated that the therapeutic spectrum of IGF-I is quite different from that of GH.

Recombinant human insulin-like growth factor I stimulates growth and has distinct effects on organ size in hypophysectomized rats.

Recombinant human insulin-like growth factor I (rhIGF-I) was infused subcutaneously into hypophysectomized rats for as long as 18 days. Three hundred micrograms (39 nmol) of rhIGF-I per day and 200 milliunits (4.5 nmol) of human growth hormone (hGH) per day increased body weight, tibial epiphyseal width, longitudinal bone growth, and trabecular bone formation similarly. Weight gains of the kidneys and spleen, however, were greater with rhIGF-I than with hGH, whereas the weight of the epididymal fat pads was reduced with rhIGF-I. The weight of the thymus was increased by rhIGF-I treatment. Thus, IGF-I administered over a prolonged period of time mimics GH effects in hypophysectomized rats. Quantitative differences between rhIGF-I and hGH treatment with respect to organ weights may be related to different forms of circulating IGF-I or may be due to independent effects of GH and IGF-I. The results support the somatomedin hypothesis, but they also stress the role of GH as a modulator of IGF-I action.