Effect of a Novel Long-Acting Neutralizing Monoclonal ACTH Antibody (ALD1611) in the Neonatal Rat: Basal and Corticosterone Responses to ACTH and Hypoxic Stress
Background: The control of steroidogenesis in the neonatal adrenal gland is of great clinical interest. We have demonstrated that the newborn rat [postnatal day (PD) 2] exhibits a corticosterone response to hypoxia in the absence of an increase in plasma ACTH measured by radioimmunoassay, whereas the corticosterone response to exogenous ACTH is intact. By PD8, the corticosterone response to hypoxia is ACTH-dependent. This apparently ACTH-independent response to hypoxia in the newborn rat may be due to an increase in a bioactive, non-immunoassayable form of ACTH.
Purpose: To evaluate the ACTH-independent response to hypoxia in newborn rats using a novel, specific neutralizing antibody (ALD1611) to ACTH.
Methods: Rat pups (N = 6–14 per group) were given ALD1611 (20 mg/kg, intraperitoneal) or vehicle on the morning of PD1, PD7, or PD14; 24 hours later (on PD2, PD8, or PD15), baseline blood samples and adrenal glands were obtained. Then, porcine ACTH [1-39] was injected (20 mcg/kg, subcutaneous) or hypoxia (8% O2) was administered, and blood was sampled for ACTH and corticosterone and adrenal glands collected for quantitative polymerase chain reaction (qPCR) 60 minutes later.
Results: Treatment with ALD1611 decreased baseline corticosterone and eliminated the corticosterone response to ACTH or hypoxia in all age groups. This occurred despite the fact that hypoxia did not induce a statistically significant increase in plasma ACTH in the PD2 pups. In additional experiments, we found that the magnitude and duration of the attenuation of the adrenal response to ACTH injection or hypoxia was related to the dose of ALD1611 (0.1 mg/kg to 20 mg/kg, intraperitoneal). ALD1611 also blocked stress-induced changes in the expression of mRNAs (by qPCR) of critical adrenal steroidogenic pathway genes.
Conclusion: ALD1611 is highly effective in decreasing basal plasma corticosterone and in blocking the adrenocortical response to exogenous ACTH and hypoxic stress in the neonatal rat. We conclude that, despite the minimal increase in plasma ACTH, the adrenal response to hypoxia is ACTH-dependent at all age groups, suggesting stress-induced increases in alternate biologically active forms of ACTH and/or hypoxia-induced increase in adrenal sensitivity in the PD2 neonatal rat. ALD1611 may prove useful in attenuating ACTH-dependent adrenal steroidogenesis in vivo and eventually in treated patients with ACTH-dependent Cushing’s syndrome.