Translational mixed-effects PKPD modelling of recombinant human growth hormone - from hypophysectomized rat to patients

Guidance for the treatment of adult growth hormone deficiency with somapacitan, a long-acting growth hormone preparation

Adult growth hormone deficiency (AGHD) is a rare endocrine disorder characterized by an abnormal body composition, metabolic abnormalities associated with increased cardiovascular diseases, bone loss, and impaired quality of life. Daily subcutaneous injections with recombinant growth hormone (GH) can alleviate the abnormalities associated with AGHD. Several long-acting GH (LAGH) preparations are currently in development that aim to reduce treatment burden for patients receiving daily GH injections. Somapacitan (Sogroya^®; Novo Nordisk, Denmark) is the first LAGH preparation that has been approved for treatment of AGHD in the United States, Europe, and Japan. The recent approval of somapacitan and anticipated approval of other LAGH molecules presents new questions for physicians planning to treat AGHD with LAGH in the future. Differences in the technologies used to prolong the half-life of recombinant GH are expected to result in variations in pharmacokinetic and pharmacodynamic profiles between preparations. Therefore, it is essential that physicians understand and consider such variations when treating patients with these novel GH replacement therapies. Here, we present a set of treatment recommendations that have been created to guide physicians initiating therapy with somapacitan in patients with AGHD who are eligible for GH replacement. Furthermore, we will review the published data that underlie these recommendations to explain the rationale for the treatment and monitoring advice provided.

Effect of Kidney or Hepatic Impairment on the Pharmacokinetics and Pharmacodynamics of Somapacitan: Two Open-Label, Parallel-Group Trials

INTRODUCTION

Somapacitan is a long-acting growth hormone (GH) derivative being developed for once-weekly dosing in patients with GH deficiency (GHD). Our objective was to evaluate the impact of kidney or hepatic impairment on somapacitan exposure in adults.

METHODS

In two open-label, parallel-group, single-center, 6-week trials, eligible subjects (18-75 years of age, body mass index 18.5-34.9 kg/m², GH-naïve, without GHD) were divided into five kidney (total n = 44) or three hepatic (n = 34) function groups. Subjects with normal kidney/hepatic function were matched to those with kidney/hepatic impairment by age, sex, and body weight. Subjects received three subcutaneous somapacitan administrations (0.08 mg/kg) on days 1, 8, and 15. Blood samples were collected before each dose, at 28 time points throughout 2 weeks after the last dose, and at follow-up (3-4 weeks after the last dose). The primary endpoint was area under the somapacitan serum concentration-time curve up to 1 week after the last dose (AUC_0-168 h), while secondary endpoints included AUC_0-168 h of insulin-like growth factor (IGF)-I.

RESULTS

In the kidney impairment trial, somapacitan AUC_0-168 h was higher in groups with severe kidney impairment and requiring hemodialysis versus the normal kidney function group (estimated ratio and 90% confidence interval 1.75 [1.00-3.06] and 1.63 [1.01-2.61], respectively). AUC_0-168 h of IGF-I was increased in the moderate impairment group (1.35 [1.09-1.66]), severe impairment group (1.40 [1.10-1.78]), and requiring hemodialysis group (1.24 [1.01-1.52]), compared with the normal function group. In the hepatic impairment trial, somapacitan AUC_0-168 h was significantly higher in the moderate impairment group compared with the normal hepatic function group (4.69 [2.92-7.52]). IGF-I AUC_0-168 h was lower in both hepatic impairment groups (0.85 [0.67-1.08] for the mild impairment group and 0.75 [0.60-0.95] for the moderate impairment group) compared with the normal function group. No new safety or tolerability issues were observed.

CONCLUSIONS

In summary, somapacitan exposure increased with level of kidney/hepatic impairment. Clinically, this will be taken into account when treating adults with GHD with somapacitan, as doses should be individually titrated.

CLINICAL TRIAL REGISTRATION

NCT03186495 (kidney impairment trial, registered 12 June 2017); NCT03212131 (hepatic impairment trial, registered 30 June 2017).

Tissue Distribution and Receptor Activation by Somapacitan, a Long Acting Growth Hormone Derivative

Somapacitan is a long-acting, once-weekly, albumin-binding growth hormone (GH) derivative. The reversible albumin-binding properties leads to prolonged circulation half-life. Here, we investigated and compared somapacitan with human GH on downstream receptor signaling in primary hepatocytes and hepatocellular models and using isothermal titration calorimetry to characterize receptor binding of somapacitan in the presence or absence of human serum albumin (HSA). With non-invasive fluorescence imaging we quantitatively visualize and compare the temporal distribution and examine the tissue-specific growth hormone receptor (GHR) activation at distribution sites. We found that signaling kinetics were slightly more rapid and intense for GH compared with somapacitan. Receptor binding isotherms were characterized by a high and a low affinity interaction site with or without HSA. Using in vivo optical imaging we found prolonged systemically biodistribution of somapacitan compared with GH, which correlated with plasma pharmacokinetics. Ex vivo mouse organ analysis revealed that the temporal fluorescent intensity in livers dosed with somapacitan was significantly increased compared with GH-dosed livers and correlated with the degree of downstream GHR activation. Finally, we show that fluorescent-labeled analogs distributed to the hypertrophic zone in the epiphysis of proximal tibia of hypophysectomized rats and that somapacitan and GH activate the GHR signaling in epiphyseal tissues.

Towards a Göttingen minipig model of adult onset growth hormone deficiency: evaluation of stereotactic electrocoagulation method

Background

Adult onset growth hormone (GH) deficiency (AGDH) is a potentially underdiagnosed condition, caused by damage to the pituitary gland. AGHD is treated with growth hormone replacement therapy. A large variety of clinical symptoms and changes in the metabolic homeostasis can be observed and quantified. New large animal models are needed for future drug development.

New method

In this study, we evaluate methods for a new large non-primate animal model of GH deficiency in post pubertal Göttingen Minipigs (minipig). Lesions in the pituitary gland were made by stereotaxic monopolar thermo-coagulation guided by magnetic resonance imaging (MRI), and pituitary function was evaluated using insulin tolerance test (ITT) with measurements of growth hormone secretion induced by hypoglycemia.

Results

Lesions were successfully applied to the pituitary gland without any damage to surrounding tissue including the hypothalamus, which was confirmed by post-operative MRI and post mortem histology. Plasma levels of GH during ITT showed no decrease in secreted levels one week after surgery compared to levels obtained before surgery.

Comparison with existing methods

Compared to other GH insufficiency models, eloquent brain tissue is spared. Furthermore, alternatively to rodent models, a large animal model would allow the use of human intended equipment to evaluate disease. Using the minipig avoids social, economical and ethical issues, compared with primates.

Conclusion

The lesions did not remove all GH production, but proof of concept is demonstrated. In addition, the ITT is presented as a safe and efficient method to diagnose GH deficiency in minipigs.

Reaction Kinetic Models of Antibiotic Heteroresistance

Bacterial heteroresistance (i.e., the co-existence of several subpopulations with different antibiotic susceptibilities) can delay the clearance of bacteria even with long antibiotic exposure. Some proposed mechanisms have been successfully described with mathematical models of drug-target binding where the mechanism's downstream of drug-target binding are not explicitly modeled and subsumed in an empirical function, connecting target occupancy to antibiotic action. However, with current approaches it is difficult to model mechanisms that involve multi-step reactions that lead to bacterial killing. Here, we have a dual aim: first, to establish pharmacodynamic models that include multi-step reaction pathways, and second, to model heteroresistance and investigate which molecular heterogeneities can lead to delayed bacterial killing. We show that simulations based on Gillespie algorithms, which have been employed to model reaction kinetics for decades, can be useful tools to model antibiotic action via multi-step reactions. We highlight the strengths and weaknesses of current models and Gillespie simulations. Finally, we show that in our models, slight normally distributed variances in the rates of any event leading to bacterial death can (depending on parameter choices) lead to delayed bacterial killing (i.e., heteroresistance). This means that a slowly declining residual bacterial population due to heteroresistance is most likely the default scenario and should be taken into account when planning treatment length.

Translational mixed-effects PKPD modelling of recombinant human growth hormone - from hypophysectomized rat to patients

BACKGROUND AND PURPOSE

We aimed to develop a mechanistic mixed-effects pharmacokinetic (PK)-pharmacodynamic (PD) (PKPD) model for recombinant human growth hormone (rhGH) in hypophysectomized rats and to predict the human PKPD relationship.

EXPERIMENTAL APPROACH

A non-linear mixed-effects model was developed from experimental PKPD studies of rhGH and effects of long-term treatment as measured by insulin-like growth factor 1 (IGF-1) and bodyweight gain in rats. Modelled parameter values were scaled to human values using the allometric approach with fixed exponents for PKs and unscaled for PDs and validated through simulations relative to patient data.

KEY RESULTS

The final model described rhGH PK as a two compartmental model with parallel linear and non-linear elimination terms, parallel first-order absorption with a total s.c. bioavailability of 87% in rats. Induction of IGF-1 was described by an indirect response model with stimulation of kin and related to rhGH exposure through an Emax relationship. Increase in bodyweight was directly linked to individual concentrations of IGF-1 by a linear relation. The scaled model provided robust predictions of human systemic PK of rhGH, but exposure following s.c. administration was over predicted. After correction of the human s.c. absorption model, the induction model for IGF-1 well described the human PKPD data.

CONCLUSIONS

A translational mechanistic PKPD model for rhGH was successfully developed from experimental rat data. The model links a clinically relevant biomarker, IGF-1, to a primary clinical end-point, growth/bodyweight gain. Scaling of the model parameters provided robust predictions of the human PKPD in growth hormone-deficient patients including variability.