Romosozumab and antiresorptive treatment: the importance of treatment sequence

To evaluate whether treatment sequence affects romosozumab response, this analysis reviewed studies where romosozumab was administered before or following an antiresorptive (alendronate or denosumab). Initial treatment with romosozumab followed by an antiresorptive resulted in larger increases in bone mineral density of both hip and spine compared with the reverse sequence.

INTRODUCTION

Teriparatide followed by an antiresorptive increases bone mineral density (BMD) more than using an antiresorptive first. To evaluate whether treatment sequence affects romosozumab response, we reviewed randomized clinical trials where romosozumab was administered before (ARCH, FRAME) or following (STRUCTURE, Phase 2 extension) an antiresorptive (alendronate or denosumab, respectively).

METHODS

We evaluated BMD percentage change for total hip (TH) and lumbar spine (LS) and response rates (BMD gains ≥ 3% and ≥ 6%) at years 1 and 2 (except STRUCTURE with only 1-year data available).

RESULTS

With 1-year romosozumab initial therapy in ARCH and FRAME, TH BMD increased 6.2% and 6.0%, and LS BMD increased 13.7% and 13.1%, respectively. When romosozumab was administered for 1 year after alendronate (STRUCTURE) or denosumab (Phase 2 extension), TH BMD increased 2.9% and 0.9%, respectively, and LS BMD increased 9.8% and 5.3%, respectively. Over 2 years, TH and LS BMD increased 7.1% and 15.2% with romosozumab/alendronate, 8.5% and 16.6% with romosozumab/denosumab, and 3.8% and 11.5% with denosumab/romosozumab, respectively. A greater proportion of patients achieved BMD gains ≥ 6% when romosozumab was used first, particularly for TH, versus the reverse sequence (69% after romosozumab/denosumab; 15% after denosumab/romosozumab).

CONCLUSION

In this study, larger mean BMD increases and greater BMD responder rates were achieved when romosozumab was used before, versus after, an antiresorptive agent. Since BMD on treatment is a strong surrogate for bone strength and fracture risk, this analysis supports the thesis that initial treatment with romosozumab followed by an antiresorptive will result in greater efficacy versus the reverse sequence.

Romosozumab Enhances Vertebral Bone Structure in Women With Low Bone Density

Romosozumab monoclonal antibody treatment works by binding sclerostin and causing rapid stimulation of bone formation while decreasing bone resorption. The location and local magnitude of vertebral bone accrual by romosozumab and how it compares to teriparatide remains to be investigated. Here we analyzed the data from a study collecting lumbar computed tomography (CT) spine scans at enrollment and 12 months post-treatment with romosozumab (210 mg sc monthly, n = 17), open-label daily teriparatide (20 μg sc, n = 19), or placebo (sc monthly, n = 20). For each of the 56 women, cortical thickness (Ct.Th), endocortical thickness (Ec.Th), cortical bone mineral density (Ct.bone mineral density (BMD)), cancellous BMD (Cn.BMD), and cortical mass surface density (CMSD) were measured across the first lumbar vertebral surface. In addition, color maps of the changes in the lumbar vertebrae structure were statistically analyzed and then visualized on the bone surface. At 12 months, romosozumab improved all parameters significantly over placebo and resulted in a mean vertebral Ct.Th increase of 10.3% versus 4.3% for teriparatide, an Ec.Th increase of 137.6% versus 47.5% for teriparatide, a Ct.BMD increase of 2.1% versus a -0.1% decrease for teriparatide, and a CMSD increase of 12.4% versus 3.8% for teriparatide. For all these measurements, the differences between romosozumab and teriparatide were statistically significant (p < 0.05). There was no significant difference between the romosozumab-associated Cn.BMD gains of 22.2% versus 18.1% for teriparatide, but both were significantly greater compared with the change in the placebo group (-4.6%, p < 0.05). Cortical maps showed the topographical locations of the increase in bone in fracture-prone areas of the vertebral shell, walls, and endplates. This study confirms widespread vertebral bone accrual with romosozumab or teriparatide treatment and provides new insights into how the rapid prevention of vertebral fractures is achieved in women with osteoporosis using these anabolic agents. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Romosozumab reduces incidence of new vertebral fractures across severity grades among postmenopausal women with osteoporosis

Vertebral fractures (VFs) are the most common type of osteoporotic fracture, and their prevalence and severity are key risk factors for future fragility fractures. Here, we assess the treatment effect of romosozumab on the incidence of new on-study VFs according to Genant severity grades (mild, moderate, and severe). Data are reported from two phase 3 clinical studies for patients who received romosozumab versus placebo through 12 months, followed by denosumab through 24 months (FRAME: NCT01575834), and for patients who received romosozumab through 12 months, followed by alendronate through 24 months, versus alendronate only through 24 months (ARCH: NCT01631214). The treatment effect of romosozumab is reported for all included patients, and for patients with prevalent and severe baseline VFs. The incidence of new moderate-or-severe VFs was reduced through 12 months for patients treated with romosozumab versus placebo (FRAME; 0.25% versus 1.42%, respectively; p < 0.001) or alendronate (ARCH; 2.78% versus 4.00%, respectively; p = 0.042). Furthermore, the treatment effect of romosozumab on the incidence of new VFs across moderate and severe severity grades was independent of baseline VF prevalence or severity; through 12 months, consistent reductions in new moderate-or-severe VFs were observed regardless of prevalent (FRAME; p = 0.18) or severe (ARCH; p = 0.52) VFs at baseline. Reductions in the incidence of new moderate and severe VFs were sustained through 24 months, after transition from romosozumab to denosumab or alendronate, independent of baseline VF prevalence or severity; no significant interactions were observed between the incidence of new moderate-or-severe VFs and the presence of prevalent (FRAME; p = 0.81) or severe (ARCH; p = 0.99) VFs at baseline. With increasing recommendations for initial treatment with bone-forming agents for postmenopausal women with osteoporosis, these analyses will help to inform treatment decisions for patients at very high risk of VF.

A single dose of zoledronate preserves bone mineral density for up to 2 years after a second course of romosozumab

This phase 2 study evaluated the efficacy and safety of transitioning to zoledronate following romosozumab treatment in postmenopausal women with low bone mass. A single dose of 5 mg zoledronate generally maintained the robust BMD gains accrued with romosozumab treatment and was well tolerated.

INTRODUCTION

Follow-on therapy with an antiresorptive agent is necessary to maintain the skeletal benefits of romosozumab therapy. We evaluated the use of zoledronate following romosozumab treatment.

METHODS

This phase 2, dose-finding study enrolled postmenopausal women with low bone mineral density (BMD). Subjects who received various romosozumab doses or placebo from months 0-24 were rerandomized to denosumab (60 mg SC Q6M) or placebo for 12 months, followed by open-label romosozumab (210 mg QM) for 12 months. At month 48, subjects who had received active treatment for 48 months were assigned to no further active treatment and all other subjects were assigned to zoledronate 5 mg IV. Efficacy (BMD, P1NP, and β-CTX) and safety were evaluated for 24 months, up to month 72.

RESULTS

A total of 141 subjects entered the month 48-72 period, with 51 in the no further active treatment group and 90 in the zoledronate group. In subjects receiving no further active treatment, lumbar spine (LS) BMD decreased by 10.8% from months 48-72 but remained 4.2% above the original baseline. In subjects receiving zoledronate, LS BMD was maintained (percentage changes: - 0.8% from months 48-72; 12.8% from months 0-72). Similar patterns were observed for proximal femur BMD in both groups. With no further active treatment, P1NP and β-CTX decreased but remained above baseline at month 72. Following zoledronate, P1NP and β-CTX levels initially decreased but approached baseline by month 72. No new safety signals were observed.

CONCLUSION

A zoledronate follow-on regimen can maintain robust BMD gains achieved with romosozumab treatment.

T-Score as an Indicator of Fracture Risk During Treatment With Romosozumab or Alendronate in the ARCH Trial

In the Active-Controlled Fracture Study in Postmenopausal Women With Osteoporosis at High Risk (ARCH) clinical trial (NCT01631214), 1 year of romosozumab followed by alendronate reduced the risk of vertebral and nonvertebral fractures compared to alendronate alone in women with prevalent fracture. We performed post hoc analyses of data from patients in ARCH (romosozumab, n = 1739; alendronate, n = 1726) who had a baseline BMD measurement and received at least one open-label alendronate dose. We evaluated 1-year mean BMD and corresponding T-score changes; proportions of patients achieving T-scores > -2.5 at the total hip (TH), femoral neck (FN), and lumbar spine (LS); and group differences in fracture rates after 12 months, while all participants were on alendronate. Subsequently, we investigated the relationship between T-scores achieved at the TH, FN, and LS at 12 months and subsequent fracture incidence. At 1 year, mean change from baseline in TH BMD was 6.3% (T-score change 0.31) with romosozumab versus 2.9% (T-score change 0.15) with alendronate (p < .001). The proportion of patients with TH T-score > -2.5 increased from 34% at baseline to 55% after 1 year of romosozumab and from 32% at baseline to 44% after 1 year of alendronate. Compared with patients receiving alendronate in year 1, those receiving romosozumab had a 75% reduction in new or worsening vertebral fracture (p < .001) in year 2, and a 19% reduction in nonvertebral fracture (p = .120) and 40% reduction in hip fracture (p = .041) during the open-label period. TH and FN T-scores achieved at month 12 were associated with subsequent nonvertebral and vertebral fracture rates and the relationships were independent of treatment received. LS T-score at 12 months was associated with vertebral but not nonvertebral fracture risk. We conclude that 1 year of romosozumab leads to larger BMD gains versus alendronate, and that the T-score achieved with either therapy is related to subsequent fracture risk. These data support the use of T-score as a therapeutic target for patients with osteoporosis. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

OR13-05 Romosozumab Treatment Lowers the Incidence of New Vertebral Fractures Across All Fracture Severity Grades Among Postmenopausal Women with Osteoporosis

Vertebral fractures (VFx) are the most common type of fracture in postmenopausal osteoporosis (PMO). VFx are generally classified using the Genant grading system as mild (grade 1), moderate (grade 2), or severe (grade 3) according to their degree of compression visualized on spinal x-rays. Regardless of their severity, VFx are associated with significant morbidity and carry the highest subsequent fracture rate of any fragility fracture. We assessed the incidence of new VFx by Genant severity grade in the romosozumab (Romo) vs placebo (Pbo) or alendronate (ALN) arms of the FRAME and ARCH studies, respectively.

In FRAME, 7,180 women with PMO were randomized 1:1 to receive monthly Romo 210 mg or Pbo for 12 months followed by biannual denosumab (DMAb) 60 mg (Romo→DMAb or Pbo→DMAb) for 12 months. In ARCH, 4,093 women with PMO and ≥ 1 fracture were randomized 1:1 to receive monthly Romo 210 mg or weekly oral ALN 70 mg for 12 months followed by ALN 70 mg (Romo→ALN or ALN→ALN) for ≥ 12 months. Throughout both studies, lateral radiographs of the spine were assessed for the presence and severity (mild, moderate, or severe) of VFx using the Genant grading at baseline and after 12 and 24 months of treatment.

The incidence of new VFx was significantly lower among patients who received Romo during the 12-month double-blind treatment phase in both studies. Over 12 months, the incidence of new VFx was 0.5% Romo vs 1.8% Pbo (P&lt;0.001) in FRAME and 3.2% Romo vs 5.0% ALN (P=0.008) in ARCH. Over 24 months, the incidence of new VFx was 0.6% Romo→DMAb vs 2.5% Pbo→DMAb (P&lt;0.001) in FRAME and 4.1% Romo→ALN vs 8.0% ALN→ALN (P&lt;0.001) in ARCH. Fewer new VFx were observed in the Romo arm of both studies across all fracture severity grades. Specifically, in FRAME, the incidence of mild VFx was 0.2% Romo vs 0.4% Pbo over 12 months and 0.2% Romo→DMAb vs 0.6% Pbo→DMAb over 24 months; the incidence of moderate VFx was 0.1% Romo vs 0.9% Pbo over 12 months and 0.2% Romo→DMAb vs 1.4% Pbo→DMAb over 24 months; and the incidence of severe VFx was 0.2% Romo vs 0.5% Pbo over 12 months and 0.2% Romo→DMAb vs 0.6% Pbo→DMAb over 24 months. Similarly, in ARCH, the incidence of mild VFx was 0.5% Romo vs 1.0% ALN over 12 months and 0.4% Romo→ALN vs 1.4% ALN→ALN over 24 months; the incidence of moderate VFx was 1.3% Romo vs 2.1% ALN over 12 months and 1.8% Romo→ALN vs 3.4% ALN→ALN over 24 months; and the incidence of severe VFx was 1.5% Romo vs 1.9% ALN over 12 months and 1.9% Romo→ALN vs 3.3% ALN→ALN over 24 months.

In conclusion, Romo administered over 12 months to women with PMO resulted in reductions in VFx across all fracture severity grades compared with Pbo and standard-of-care ALN. The treatment effect of Romo continued after patients transitioned to an antiresorptive agent. These data will help to foster treatment decisions in postmenopausal women at high risk for VFx.

MON-378 Efficacy and Safety of Romosozumab vs Alendronate Is Similar Across Different Levels of Renal Function Among Postmenopausal Women with Osteoporosis

Postmenopausal women with osteoporosis may also have renal insufficiency. We conducted a post hoc analysis of the ARCH study to determine the efficacy and safety of romosozumab (Romo) vs alendronate (ALN) among patients with different levels of baseline renal function.

In ARCH, 4,093 postmenopausal women, 55–90 years old, were randomized 1:1 to receive monthly subcutaneous Romo 210 mg or weekly oral ALN 70 mg for 12 months (double-blind phase [DBP]). Eligible patients had a bone mineral density (BMD) T score of ≤ –2.5 at the total hip (TH) or femoral neck (FN) and either ≥ 1 moderate/severe vertebral fracture (VFx) or ≥ 2 mild VFx; or a T score of ≤ –2.0 at the TH or FN and either ≥ 2 moderate/severe VFx or an Fx of the proximal femur sustained 3–24 months before randomization. Pts were excluded for significantly impaired renal function (eGFR < 35 mL/min/1.73 m², calculated using the MDRD equation). For the current analysis, patients were categorized by baseline eGFR: normal renal function (eGFR ≥ 90), mild renal insufficiency (eGFR 60–89), or moderate renal insufficiency (eGFR 30–59). The least squares mean (LSM) % change from baseline in BMD at the lumbar spine (LS), TH, and FN; incidence of new VFx; incidence of adverse events (AEs); and changes in renal function were assessed for each eGFR category at month 12 of the DBP.

At baseline, 15% of patients had eGFR ≥ 90, 60% had eGFR 60–89, 24% had eGFR 30–59, and 0.3% had eGFR 15–29. In the overall patient population, LSM % change (95% CI) from baseline in BMD (Romo vs ALN) was 13.7% (13.4–14.0) vs 5.0% (4.7–5.2) for LS, 6.2% (5.9–6.4) vs 2.8% (2.7–3.0) for TH, and 4.9% (4.7–5.2) vs 1.7% (1.5–2.0) for FN (P < 0.001 at each site). Changes in BMD were similar irrespective of baseline eGFR. Among patients with eGFR ≥ 90, eGFR 60–89, and eGFR 30–59, the incidence of new VFx (Romo/ALN) was 3.3%/7.3% (relative risk reduction [RRR] = 57%; 95% CI: 1–81), 3.2%/3.9% (RRR = 19%; 95% CI: -28–49), and 3.4%/6.2% (RRR = 51%; 95% CI: 5–75), respectively. The incidences of AEs, serious AEs, and fatal AEs were similar in both treatment groups within each eGFR category as well as across eGFR categories; there was a higher incidence of positively adjudicated cardiovascular events in the Romo vs ALN group overall and across eGFR categories. One patient in the Romo group with eGFR 60–89 at baseline and 1 in the ALN group with eGFR ≥ 90 at baseline had an AE of mild hypocalcemia. Similar percentages of patients in the Romo and ALN groups had changes in renal function over 12 months of treatment.

In conclusion, the efficacy and safety of Romo vs ALN was similar across different levels of renal function.

Glycaemic control and hypoglycaemia during 12 months of randomized treatment with insulin glargine 300 U/mL versus glargine 100 U/mL in people with type 1 diabetes (EDITION 4)

AIMS

Insulin glargine 300 U/mL (Gla-300) offers a flatter pharmacodynamic profile than insulin glargine 100 U/mL (Gla-100). We have compared these insulins over 1 year in people with type 1 diabetes (T1DM).

METHODS

EDITION 4 was a 6-month, multicentre, randomized, open-label phase 3 study. People with T1DM who completed the 6 months continued randomized Gla-300 or Gla-100 once daily, morning or evening, for a further 6 months.

RESULTS

Among 549 participants randomized, 444 completed the 12-month study period (Gla-300, 80%; Gla-100, 82%). Mean HbA1c decreased similarly from baseline to month 12 in the 2 treatment groups (difference, 0.02 [95% CI, -0.13 to 0.17]) %-units [0.2 (-1.5 to 1.9) mmol/mol]), to a mean of 7.86 %-units (62.4 mmol/mol) in both groups. For morning vs evening injection, there was no difference in HbA1c change over 12 months for Gla-100, but a significantly larger decrease in HbA1c was observed in the Gla-300 morning group than in the Gla-300 evening group (difference, -0.25 [-0.47 to -0.04] %-units [-2.7 (-5.2 to -0.4) mmol/mol]). Mean glucose from the 8-point SMPG profiles decreased from baseline, and was similar between the 2 treatment groups. Basal insulin dose was 20% higher with Gla-300 than with Gla-100, while hypoglycaemia event rates, analysed at night, over 24 hours, or according to different glycaemic thresholds, did not differ between treatment groups, regardless of injection time. Adverse event profiles did not differ between groups.

CONCLUSIONS

In T1DM, Gla-300 provides glucose control comparable to that of Gla-100, and can be given at any time of day.

Efficacy and Safety of Biosimilar SAR342434 Insulin Lispro in Adults with Type 1 Diabetes Also Using Insulin Glargine-SORELLA 1 Study

BACKGROUND

SAR342434 is a biosimilar follow-on of insulin lispro-Humalog^®. This study aimed to show similar efficacy, safety, and immunogenicity of SAR342434 (SAR-Lis) versus insulin lispro-Humalog (Ly-Lis) in adult patients with type 1 diabetes (T1DM) treated with multiple daily injections while using basal insulin glargine (Lantus^®; GLA-100).

MATERIALS AND METHODS

SORELLA-1 was a randomized, open-label phase 3 study (NCT02273180). Patients completing the 6-month main study continued on SAR-Lis or Ly-Lis, as randomized, for a 6-month safety extension. Assessments included change in HbA_1c, fasting plasma glucose (FPG), seven-point self-monitored plasma glucose (SMPG) profiles, hypoglycemic events, treatment-emergent adverse events (TEAEs), and anti-insulin antibodies (AIAs).

RESULTS

Five hundred seven patients were randomized (SAR-Lis n = 253; Ly-Lis n = 254). Least square (LS) mean (SEM) change in glycosylated hemoglobin (HbA1c) (baseline to week 26; primary endpoint) was similar in both treatment groups (SAR-Lis: -0.42% [0.051]; Ly-Lis: -0.47% [0.050]). Noninferiority at prespecified 0.3% noninferiority margin and inverse noninferiority were demonstrated (LS mean difference of SAR-Lis vs. Ly-Lis: 0.06% [95% confidence interval: -0.084 to 0.197]). At week 52 (end of extension period) versus week 26, a small HbA1c increase was observed in both groups. FPG and seven-point SMPG profile changes, including postprandial glucose excursions, were similar between groups. At week 52, similar changes in mean daily mealtime and basal insulin doses were observed. Hypoglycemia, TEAEs, and AIAs (incidence, prevalence) did not differ between groups.

CONCLUSIONS

Results from this controlled study in patients with T1DM also using GLA-100 support similar efficacy and long-term safety (including immunogenicity) of SAR-Lis and Ly-Lis.

Achieve control: a pragmatic clinical trial of insulin glargine 300 U/mL versus other basal insulins in insulin-naïve patients with type 2 diabetes

OBJECTIVE

This study aims to compare the effectiveness of insulin glargine 300 U/mL (Gla-300) with its accompanying patient support program with that of other basal insulin and available patient support programs in patients with type 2 diabetes (T2D) in a real-world setting in terms of achieving HEDIS (Healthcare Effectiveness Data and Information Set) individualized glycemic targets without documented symptomatic hypoglycemia.

METHODS

Achieve Control is a US-based, multicenter, randomized, open-label, active-controlled, parallel group pragmatic Phase IV trial in insulin-naïve patients with T2D uncontrolled on ≥2 oral antidiabetes drugs (OAD) and/or glucagon-like peptide-1 receptor antagonists (GLP-1 RA). Inclusion criteria include a diagnosis of T2D, age ≥18 years, and glycated hemoglobin (HbA1c) between 8.0% and 11.0%. Patients will be assigned to either the Gla-300 or other basal insulin group. The primary end point is the proportion of patients achieving HEDIS HbA1c targets (<8.0% [64 mmol/mol] in patients with comorbidities or aged ≥65 years; <7.0% [58 mmol/mol] in all other patients) without occurrence of symptomatic hypoglycemia (blood glucose ≤70 mg/dL) from baseline to 6 months. Secondary end points include rates of documented symptomatic nocturnal hypoglycemia and severe hypoglycemia; change from baseline in HbA1c, fasting glucose, and body weight; treatment persistence; patient-reported outcomes; and healthcare resource utilization. Planned enrollment is 3270 patients across approximately 400 clinical sites.

CONCLUSION

Pragmatic clinical trials offer the potential to assess comparative effectiveness in broadly based patient populations receiving care (with or without a corresponding educational support program) in real-world clinical settings. The results of Achieve Control should elucidate the benefits of management of T2D with Gla-300 versus other basal insulins in terms of patient outcomes, experiences, and perceptions, and its impact on healthcare resource utilization and cost.

CLINICAL TRIAL REGISTRATION

www.clinicaltrials.gov identifier is NCT02451137.

New Insulin Glargine 300 Units/mL Versus Glargine 100 Units/mL in People With Type 1 Diabetes: A Randomized, Phase 3a, Open-Label Clinical Trial (EDITION 4)

OBJECTIVE

Insulin therapy in type 1 diabetes still provides suboptimal outcomes. Insulin glargine 300 units/mL (Gla-300), with a flatter pharmacodynamic profile compared with insulin glargine 100 units/mL (Gla-100), is an approach to this problem.

RESEARCH DESIGN AND METHODS

People with type 1 diabetes, using a mealtime and basal insulin regimen, were randomized open-label to Gla-300 or Gla-100 and to morning or evening injection, continuing the mealtime analog, and followed for 6 months.

RESULTS

Participants (n = 549) were a mean age of 47 years and had a mean duration of diabetes of 21 years and BMI of 27.6 kg/m(2). The change in HbA1c (primary end point; baseline 8.1%) was equivalent in the two treatment groups (difference, 0.04% [95% CI -0.10 to 0.19]) (0.4 mmol/mol [-1.1 to 2.1]), and Gla-300 was thus noninferior. Similar results with wider 95% CIs were found for morning and evening injection times and for prebreakfast self-measured plasma glucose (SMPG) overall. Results were also similar for Gla-300 when morning and evening injection time was compared, including overlapping 8-point SMPG profiles. Hypoglycemia did not differ, except for the first 8 weeks of the study, when nocturnal confirmed or severe hypoglycemia was lower with Gla-300 (rate ratio 0.69 [95% CI 0.53-0.91]). Hypoglycemia with Gla-300 did not differ by time of injection. The basal insulin dose was somewhat higher at 6 months for Gla-300. The adverse event profile did not differ and was independent of the Gla-300 time of injection. Weight gain was lower with Gla-300.

CONCLUSIONS

In long-duration type 1 diabetes, Gla-300 provides similar glucose control to Gla-100, with a lower risk of hypoglycemia after transfer from other insulins, independent of time of injection, and less weight gain.

Receptors for insulin-like growth factors I and II: autoradiographic localization in rat brain and comparison to receptors for insulin

Receptors for insulin-like growth factor I (IGF-I) in rat brain were visualized using autoradiography with [125I]IGF-I. The binding of the labeled peptide was competed for fully by high concentrations of unlabeled IGF-I. At intermediate concentrations of unlabeled peptide the binding of [125I]IGF-I was competed for by unlabeled IGF-I more effectively than by IGF-II or insulin, which is typical of receptors for IGF-I. Essentially every brain section shows specific binding of IGF-I, and the pattern of binding of IGF-I to its receptors correlated well with the cytoarchitectonic structures. In parallel studies we showed that [125I]IGF-II was bound to tissue sections of rat brain and that the binding was competed for by an excess of unlabeled IGF-II. However, intermediate concentrations of unlabeled peptides gave inconclusive results. To confirm that the binding of [125I]IGF-II was to IGF-II receptors, we showed that antibodies specific for the IGF-II receptor inhibited the binding of labeled IGF-II. Furthermore, the binding of the antibody to regions of the brain section, visualized by the application of [125I]protein-A, gave patterns indistinguishable from those obtained with [125I]IGF-II alone. Again, the binding was very widely distributed throughout the central nervous system, and the patterns of distribution corresponded well to the underlying neural structures. Densitometric analysis of the receptors enabled us to compare the distribution of IGF-I receptors with that of IGF-II receptors as well as retrospectively with that of insulin receptors.