Rationale GDF11 (Growth Differentiation Factor 11) is a member of the transforming growth factor β (TGFβ) super family of secreted factors. A recent study showed that reduced GDF11 blood levels with aging was associated with pathological cardiac hypertrophy (PCH), and restoring GDF11 to normal levels in old mice rescued PCH. Objective To determine if and by what mechanism GDF11 rescues aging dependent PCH. Methods and Results 24-month-old C57BL/6 mice were given a daily injection of either recombinant (r) GDF11 at 0.1mg/kg or vehicle for 28 days. rGDF11 bioactivity was confirmed in-vitro. After treatment, rGDF11 levels were significantly increased but there was no significant effect on either heart weight (HW) or body weight (BW). HW/BW ratios of old mice were not different from 8 or 12 week-old animals, and the PCH marker ANP was not different in young versus old mice. Ejection fraction, internal ventricular dimension, and septal wall thickness were not significantly different between rGDF11 and vehicle treated animals at baseline and remained unchanged at 1, 2 and 4 weeks of treatment. There was no difference in myocyte cross-sectional area rGDF11 versus vehicle-treated old animals. In vitro studies using phenylephrine-treated neonatal rat ventricular myocytes (NRVM), to explore the putative anti-hypertrophic effects of GDF11, showed that GDF11 did not reduce NRVM hypertrophy, but instead induced hypertrophy. Conclusions Our studies show that there is no age-related PCH in disease free 24-month-old C57BL/6 mice and that restoring GDF11 in old mice has no effect on cardiac structure or function.

Rationale: Possible beneficial effects of Growth Differentiation Factor 11 (GDF11) on the normal, diseased, and aging heart have been reported, including reversing aging induced hypertrophy. These effects have not been well validated. High levels of GDF11 have also been shown to cause cardiac and skeletal muscle wasting. These controversies could be resolved if dose-dependent effects of GDF11 were defined in normal and aged animals as well as in pressure overload induced pathological hypertrophy. Objective: To determine dose-dependent effects of GDF11 on normal hearts and those with pressure overload induced cardiac hypertrophy. Methods and Results: 12–13-week-old C57BL/6 mice underwent transverse aortic constriction (TAC) surgery. One-week post TAC, these mice received recombinant GDF11 at one of 3 doses: 0.5 mg/kg, 1.0 mg/kg, or 5.0 mg/kg for up to 14 days. Treatment with GDF11 increased plasma concentrations of GDF11 and p-SMAD2 in the heart. There were no significant differences in the peak pressure gradients across the aortic constriction between treatment groups at one-week post-TAC. Two weeks of GDF11 treatment caused dose-dependent decreases in cardiac hypertrophy as measured by HW/TL ratio, myocyte cross sectional area, and LV mass. GDF11 improved cardiac pump function while preventing TAC-induced ventricular dilation and caused a dose-dependent decrease in interstitial fibrosis (in vivo), despite increasing markers of fibroblast activation and myofibroblast transdifferentiation (in vitro). Treatment with the highest dose (5.0mg/kg) of GDF11 caused severe body weight loss, with significant decreases in both muscle and organ weights and death in both sham and TAC mice. Conclusions: Although GDF11 treatment can reduce pathological cardiac hypertrophy and associated fibrosis while improving cardiac pump function in pressure overload, high doses of GDF11 cause severe cachexia and death. Use of GDF11 as a therapy could have potentially devastating actions on the heart and other tissues.

Antibodies with pH-dependent binding to both target antigens and neonatal Fc receptor (FcRn) provide an alternative tool to conventional neutralizing antibodies, particularly for therapies where reduction in antigen level is challenging due to high target burden. However, the requirements for optimal binding kinetic framework and extent of pH dependence for these antibodies to maximize target clearance from circulation are not well understood. We have identified a series of naturally-occurring high affinity antibodies with pH-dependent target binding properties. By in vivo studies in cynomolgus monkeys, we show that pH-dependent binding to the target alone is not sufficient for effective target removal from circulation, but requires Fc mutations that increase antibody binding to FcRn. Affinity-enhanced pH-dependent FcRn binding that is double-digit nM at pH 7.4 and single-digit nM at pH 6 achieved maximal target reduction when combined with similar target binding affinities in reverse pH directions. Sustained target clearance below the baseline level was achieved 3 weeks after single-dose administration at 1.5 mg/kg. Using the experimentally derived mechanistic model, we demonstrate the essential kinetic interplay between target turnover and antibody pH-dependent binding during the FcRn recycling, and identify the key components for achieving maximal target clearance. These results bridge the demand for improved patient dosing convenience with the “know-how” of therapeutic modality by design.

Aim: Volumetric absorptive microsampling (VAM) is being increasingly applied in nonclinical pharmacokinetic studies. Although there are published results for VAM use in small molecule pharmacokinetics (PK) studies, there is limited data on the utility of VAM for protein therapeutics. Results: We describe the use of Mitra® microsampler for blood sampling, ELISA quantitation and PK analysis of two marketed therapeutic monoclonal antibodies administered to rat. Results generated for these monoclonal antibodies using Mitra® were compared with both serum and whole blood sampling methods in the same study. Conclusion: The low relative standard deviation among the three sets of PK data suggest that Mitra® microsampler could be useful in early nonclinical PK studies for protein therapeutics where reduction and refinement of animal use is desirable.

Loss of skeletal muscle mass and function results in loss of mobility for elderly patients. Novel therapies that can protect and/or restore muscle function during aging would have profound effects on the quality of life for this population. Growth differentiation factor 11 (GDF11) has been proposed as a "youthful" circulating factor that can restore cardiac, neural, and skeletal muscle functions in aging animals. However, conflicting data has been recently published that casts doubt on these assertions. We used a complex rat model of skeletal muscle injury that physiologically mimics injuries seen in patients; to investigate the ability of GDF11 and to enhance skeletal muscle regeneration after injury in older rats. Our data showed that GDF11 treatment resulted in a significant increase in tissue fibrosis, accompanied by attenuated functional recovery, as compared to animals treated with vehicle alone. GDF11 impaired the recovery of skeletal muscle function in older rats after injury.