A Randomized Phase 1 Study Comparing the PK, PD, Safety, and Immunogenicity of Proposed Biosimilar RGB‐14‐X and Denosumab in Healthy Adult Males

ABSTRACT

Denosumab is a monoclonal antibody targeting the receptor activator of nuclear factor kappa‐b ligand widely used for the prevention of skeletal‐related events in patients with bone metastases. This Phase 1 randomized, double‐blind, two‐arm, parallel‐group study assessed the equivalence in pharmacokinetics (PK) and compared the pharmacodynamics (PD), safety, and immunogenicity of the proposed biosimilar RGB‐14‐X and reference denosumab in healthy males. Participants were randomized 1:1 to a single subcutaneous 60 mg dose of RGB‐14‐X or reference denosumab, with 252 days of follow‐up. Primary PK endpoints were maximum observed serum concentration (C_max) and area under the concentration‐time curve from time 0 to last quantifiable concentration (AUC_0‐last) and extrapolated to infinity (AUC_0‐inf). Secondary objectives were to compare additional PK parameters, safety and tolerability, PD and immunogenicity between groups. Of 165 participants randomized, 162 (98.2%) completed the study. The geometric mean ratios and corresponding 90% confidence intervals of RGB‐14‐X versus reference denosumab for C_max, AUC_0‐last, and AUC_0‐inf were within the pre‐specified range of 0.80–1.25, demonstrating equivalence. No notable differences were observed in secondary PK or PD parameters between groups; maximum reduction in concentration of the bone resorption marker serum C‐terminal telopeptide of type I collagen (CTX) and the extent and duration of reduction in CTX levels over time were similar. RGB‐14‐X was well tolerated with a similar safety profile to reference denosumab. No anti‐drug or neutralizing antibodies were detected in either group. RGB‐14‐X demonstrated biosimilarity to reference denosumab, with equivalent PK and similar PD, safety, and immunogenicity outcomes in healthy males.

Study Highlights

• What is the current knowledge on the topic?

  • ○
    Denosumab is a monoclonal antibody widely used for the prevention of skeletal‐related events in patients with bone metastases. Access to denosumab may, however, be limited due to cost; therefore, alternatives are needed to facilitate access to treatment while reducing healthcare costs.
• What question did this study address?

  • ○
    This Phase 1 randomized study assessed the equivalence in PK and compared the PD, safety, and immunogenicity of RGB‐14‐X and denosumab in healthy adult males.
• What does this study add to our knowledge?

  • ○
    These results further demonstrate the equivalence of RGB‐14 and denosumab in healthy adult males, complementing the previously reported Phase 3 results in women with postmenopausal osteoporosis. This study therefore strengthens the body of evidence suggesting that RGB‐14 effectively replicates the therapeutic benefit of denosumab.
• How might this change clinical pharmacology or translational science?

  • ○
    The use of RGB‐14 has proven equivalence to the originator molecule, denosumab, and potentially provides a low‐cost alternative treatment, thereby improving access to biologic therapies and improving patient outcomes.

1. Introduction

Receptor activator of nuclear factor kappa‐b ligand (RANKL) is an essential differentiation factor for the formation, function, and survival of osteoclasts [1, 2]. Osteoclasts are key mediators in the development and progression of bone metastases, contributing to the homing of cancer cells, bone resorption, and tumor growth [1]. RANK signaling also plays a pivotal role in the pathogenesis of giant cell tumor of the bone, with secretion of RANKL by stromal cells and subsequent activation of RANK signaling on osteoclast‐like giant cells contributing to osteolysis and further expansion of the tumor [3].

Denosumab is a monoclonal antibody that binds to RANKL, inhibiting the development, function, and survival of osteoclasts, and thereby reducing osteoclastic bone resorption and cancer‐induced skeletal‐related events (SRE) [2, 4, 5, 6, 7, 8, 9, 10]. Originator denosumab is marketed as Prolia and Xgeva [11, 12, 13, 14]. Prolia at a dose of 60 mg (subcutaneous, SC) every 6 months was first approved in the United States (US) and European Union (EU) in 2010 [13, 14], for the treatment of osteoporosis and subsequently for aromatase inhibitor‐induced bone loss in early hormone receptor‐positive breast cancer (US and Switzerland) [4] as well as bone loss associated with hormone ablation in prostate cancer [6, 13, 14, 15, 16]. Xgeva at a dose of 120 mg SC every 4 weeks was first approved in the US in 2010 and then in 2011 in the EU for the prevention of SREs in adults with advanced malignancies involving bone [11, 12]; it was further approved in patients with myeloma bone disease, for the treatment of adults and skeletally mature adolescents with unresectable giant cell tumor of the bone, and the treatment of hypercalcaemia of malignancy refractory to bisphosphonate therapy (US only) [11, 12].

Denosumab is recommended by the European Society for Medical Oncology for reducing skeletal complications in metastatic bone disease and cancer treatment‐induced bone loss. It is also recommended in the American Society of Clinical Oncology‐endorsed Cancer Care Ontario guideline for men with non‐metastatic prostate cancer receiving androgen‐deprivation therapy who are at high risk of fracture [17, 18]. The efficacy and safety of denosumab 120 mg (SC, every 4 weeks [Q4W]) have been established in multiple randomized clinical trials [7, 8, 9], with superior efficacy demonstrated for denosumab compared with the bisphosphonate zoledronic acid (intravenous, Q4W) in patients with breast cancer with bone metastases, men with castration‐resistant prostate cancer [7, 8], and non‐inferior efficacy demonstrated in patients with advanced cancer (excluding breast and prostate cancer) or multiple myeloma [9]. For the treatment of giant cell tumor of bone, the safety and efficacy of a treatment scheme of denosumab 120 mg (SC, Q4W), with loading doses of 120 mg SC on Days 8 and 15 of the first cycle have been demonstrated in open‐label trials [19, 20, 21].

Despite the well‐established efficacy and safety profile of denosumab, access to this therapy may be limited by cost, particularly when cheaper alternatives, such as bisphosphonates, are available [22, 23]. Biosimilars with the same mechanism of action and equivalent efficacy and comparable safety to their originators typically enter the market at a lower price than their reference compounds [24], offering the potential to facilitate access to these therapies while reducing healthcare costs [22, 23]. Equivalence of biosimilars with reference therapeutics, including quality characteristics, biological activity, pharmacokinetics (PK), pharmacodynamics (PD), safety, and efficacy has to be demonstrated based on regulatory requirements [25, 26].

RGB‐14‐X and RGB‐14‐P are proposed biosimilars to Xgeva and Prolia, respectively, manufactured by Gedeon Richter wholly within Europe. RGB‐14‐P and RGB‐14‐X are differentiated by their concentration and indicated dose. The structural and functional similarity between RGB‐14 and denosumab has been previously shown [27]. As a single clinical program was undertaken for RGB‐14‐P and RGB‐14‐X, both products were required by the European Medicines Agency (EMA) to be included in the biosimilarity studies. Consequently, the program consisted of two key clinical trials: a Phase 1, comparative PK/PD study of Xgeva versus RGB‐14‐X and a Phase 3, comparative efficacy/safety study of Prolia versus RGB‐14‐P. Results of the Phase 3 efficacy/safety study have been published elsewhere [28]. The aim of the Phase 1 study (EudraCT: 2020‐003953‐32) was to assess the equivalence in PK and compare the PD, safety and immunogenicity of RGB‐14‐X and Xgeva (hereafter reference denosumab) in healthy adult males.

2. Methods

2.1. Study Design and Procedures

This was a Phase 1, randomized, double‐blind, two‐arm, parallel‐group study (EudraCT: 2020‐003953‐32) conducted at three study sites (one in the UK and two in Germany). Participants were randomized 1:1 (stratified by body mass index [BMI] and study site) to receive a single SC injection of 60 mg RGB‐14‐X or reference denosumab (US‐sourced Xgeva) in the abdomen. The BMI stratification groups were as follows: 19–21.99 kg/m², 22–24.99 kg/m², 25–26.99 kg/m², and 27–29 kg/m². Participants were allocated a 4‐digit randomization number, and randomization lists were prepared for each BMI group and study site; each study site received the corresponding code‐break envelopes.

In accordance with the regulatory guidance for biosimilars set forth by the US Food and Drug Administration (FDA) [25] and EMA [26, 29], to demonstrate PK equivalence, a lower, more sensitive [30], 60 mg dose was selected for this study, rather than the approved 120 mg dose of Xgeva. To minimize bias, all participants, study teams, laboratory personnel, and the sponsor's medical monitor were blinded to treatment throughout the study. Unblinded personnel included the site pharmacy personnel responsible for preparation of individual blinded subject doses, the unblinded clinical monitor and dedicated members of the clinical safety team.

The study enrolled healthy adult males (aged 28–55 years inclusive) with body weight ≥ 55 to ≤ 90 kg and BMI 19.0–29.0 kg/m² who signed informed consent. Key exclusion criteria included: prior diagnosis of metabolic bone disease or condition affecting bone metabolism; presence of, or risk factors for, osteonecrosis of the jaw; presence of hypocalcaemia or hypercalcaemia, or vitamin D deficiency; prior denosumab administration; receipt of any investigational drug (or current use of any investigational device) within the last 3 months for biologics, or within 1 month/5× elimination half‐life (whichever was longest) for small molecules. The full list of selection criteria is provided in Table S1.

Participants were admitted on Day −1, dosed on the subsequent morning of Day 1 (after an overnight fast of ≥ 8 h), and discharged on Day 2 after completion of study‐specific assessments. Outpatient follow‐up visits occurred for 252 days post discharge (on Days 3–6, 8, 10, 12, 14, 16, 21, 28, 35, every 2 weeks from Weeks 5–27, Week 31, and end of study visit at Week 36 [Day 252]). Participants received standardized supplementation of 1000 mg calcium and 800 IU vitamin D3 daily, from Day 1 to end of the study.

2.2. Ethics and IRB Approval

The study protocol, any amendments and all other relevant study documents were reviewed and approved by the independent ethics committees for each site (UK: London Brent—Research Ethics Committee, London; Germany: Ethics Committee of the Bavarian Chamber of Physicians, Munich; Landesärztekammer Baden‐Württemberg Ethik‐Kommission, Stuttgart; Protocol Amendment No. 05, Version 6.0, dated 20 June 2022, approved 11 August 2022 [Brent REC, UK] and 18 July 2022 [Munich and Stuttgart REC, Germany]). The conduct of this clinical study met all local legal and regulatory requirements and was conducted in accordance with the Declaration of Helsinki and the International Council for Harmonisation Good Clinical Practice guidelines. Signed informed consent was obtained from each participant before any study‐specific procedures.

2.3. Endpoints

The primary PK endpoints were maximum observed serum concentration (C_max); area under the concentration‐time curve (AUC) from time 0 to the time of the last quantifiable concentration (AUC_0‐last); and area under the concentration‐time curve from time 0 extrapolated to infinity (AUC_0‐inf). Secondary PK endpoints were as follows: area under the concentration‐time curve from time 0 to Day 119 (AUC_0‐119d); area under the concentration‐time curve from Day 119 to the time of the last quantifiable concentration (AUC_119d‐last); time corresponding to occurrence of C_max (t_max); and terminal elimination half‐life (t_1/2). PD endpoints were percent change from baseline (%CfB) in serum C‐terminal telopeptide of type I collagen (CTX) level; area under the effect‐time curve (AUEC) of %CfB in serum CTX; and maximum percent inhibition (I_max) of serum CTX. Safety (adverse events [AEs], serious AEs, physical examination, vital signs, electrocardiogram [ECG], laboratory assessments), injection‐site reactions, and immunogenicity (presence and titre of anti‐drug antibodies [ADAs] and neutralizing antibodies [NAbs]) were assessed.

Blood sampling for PK/PD was performed prior to dosing on Day 1; at 1 and 8 h post‐dose on Day 1 (only PK); every visit day after Day 1 for 5 days; on Day 8 and then every 2 days to Day 16; on Day 21 and then every 7 days until Day 35; on Day 49 and then every 14 days until Day 189; on Day 217; and on Day 252. Samples were taken between 7:30 a.m. and 10:00 a.m. after overnight fasting of at least 8 h. Blood sampling for immunogenicity (determination of ADAs and NAbs) was performed prior to dosing on Day 1, and on Days 14, 28, 63, 91, 119, 147, 175, 217, and 252; PK and immunogenicity samples were analyzed by the Developmental Drug Metabolism and Pharmacokinetic Department of Gedeon Richter using validated assays [31]. Serum for PD samples was analyzed by a contracted laboratory using a qualified clinical diagnostic test (Elecsys β‐Cross Laps/serum electrochemiluminescence immunoassay [Roche]).

AE monitoring was performed continuously throughout the study, from informed consent signing to end‐of‐study visit or early termination. AEs were coded using the Medical Dictionary for Regulatory Activities, version 26.0. Vital signs (pulse rate, respiratory rate, oral body temperature, and blood pressure) were evaluated at screening, admission, pre‐dose, post‐dose (1 and 8 h), and on each study visit. Laboratory tests (hematology, coagulation, clinical chemistry, and urinalysis) were performed at screening, admission, 8 h post‐dose on Day 1 (no urinalysis), and on Days 2, 8, 14, 28, 91, 147, and 252. ECG recordings were required at screening (Day −28 to −2), admission (Day −1), at pre‐dose and 8 h post‐dose on Day 1, and on Days 2, 8, 14, 28, and 252. Injection‐site assessments were performed on Day 1 pre‐dose, at 1 and 8 h post‐dose on Day 1, and on Day 2 prior to discharge.

2.4. Statistical Analyses

PK and PD parameters were calculated from individual serum concentration (PK) or CTX %CfB (PD) versus time profiles via non‐compartmental methods using validated software (Phoenix WinNonlin version 8.3). For the primary assessment of PK biosimilarity, a two one‐sided test was used, with the null hypothesis being that the test and reference were not biosimilar. For each of the primary PK parameters (C_max, AUC_0‐last, AUC_0‐inf), a linear mixed model analysis of variance was used to test the significance of the effects of treatment (treatment and site modeled as fixed factors, and BMI examined in the model as a continuous covariate). PK parameters were natural log transformed prior to analysis.

Point estimates and 90% confidence intervals (CIs) for the “test/reference” geometric mean ratios of these primary PK parameters were tabulated. Back transformation provided the ratio of geometric means and 90% CIs for these ratios. Equivalence of the primary endpoint was determined if the 90% CI for the ratio of geometric mean of RGB‐14‐X to reference denosumab was within the pre‐specified equivalence range of 0.8–1.25.

A sample size of 69 participants per arm was expected to achieve 90% power at a 5% significance level, where the true difference between the means was 5%, and inter‐percent coefficient of variation (CV%) was 35% for AUC, with equivalence limits as described above. A sample size of 172 (86 participants per arm) was originally planned. However, due to reference denosumab expiring on October 31, 2022, and the potential for increased variability from using a second batch, enrolment was stopped on October 28, 2022, after 165 participants had been dosed. The statistical approach was unchanged; the originally stipulated drop‐out rate was decreased to 18% from the original conservative 20%. No imputation of missing data was performed, except for partial dates imputation.

The safety population consists of all participants who received any dose of study drug. All outputs for AEs or treatment‐emergent AEs (TEAEs) were based on the safety analysis set, unless otherwise specified. The PK and PD populations consisted of all participants who had at least one evaluable primary PK (C_max, AUC_0‐last, and AUC_0‐inf) or PD (I_max and AUEC of %CfB in CTX) parameter and who did not have any protocol deviations that could have a relevant impact on PK parameters or CTX measurements, respectively. The immunogenicity population consists of all participants in the safety analysis set with a baseline and post‐baseline (at least one) immunogenicity assessment and those without protocol deviations that could have an impact on immunogenicity evaluations. All statistical analyses were performed using SAS Version 9.4 (SAS Institute Inc., Cary, NC, USA).

3. Results

3.1. Participant Population

Of the 609 participants screened, 165 were randomized and received study drug (RGB‐14‐X N = 83, reference denosumab N = 82) (Figure 1). Three participants discontinued from the study (one withdrew due to personal reasons, and two were withdrawn due to actual or attempted participation in other studies). The remaining 162 participants completed all study‐related assessments as per protocol. Demographics were well balanced between treatment groups. The overall mean age (standard deviation) was 39.4 (7.7) years, and most participants (77.6%) were white (Table 1).

FIGURE 1.

Participant disposition Consolidated Standards of Reporting Trials (CONSORT). ^aOne participant was withdrawn for attempting to participate in another study and one withdrew for personal reasons. ^bParticipant withdrawn from study due to parallel participation in another study.

TABLE 1.

Participant demographics (safety analysis set).

	RGB‐14‐X (N = 83)	Reference denosumab (N = 82)
Age, years, mean (SD)	39.1 (7.81)	39.8 (7.59)
Male, n (%)	83 (100)	82 (100)
Race, n (%) a
White	65 (78.3)	63 (76.8)
Black or African American	2 (2.4)	6 (7.3)
Asian	2 (2.4)	2 (2.4)
American Indian or Alaska Native	0	1 (1.2)
Other	13 (15.7)	11 (13.4)
Not reported	1 (1.2)	0
Height, cm, mean (SD)	178.0 (6.83)	178.7 (6.86)
Weight, kg, mean (SD)	78.2 (8.05)	79.0 (8.45)
BMI, kg/m 2 , mean (SD)	24.7 (2.14)	24.7 (2.13)

Abbreviations: BMI, body mass index; SD, standard deviation.

^a One participant in the reference denosumab group reported two races.

3.2. Pharmacokinetics

Serum drug concentrations over time were similar, following a single 60 mg dose of either RGB‐14‐X or reference denosumab (Figure 2). PK equivalence was demonstrated with respect to C_max, AUC_0–last, and AUC_0–inf, with 90% CI values for the ratio of geometric means falling within the 0.80–1.25 pre‐defined equivalence range (Figures 2 and S1). All secondary PK parameters (AUC_0–119d, AUC_119d‐last, t_max, t_1/2; Table 2) were also similar, with peak concentrations of RGB‐14‐X and reference denosumab occurring at a similar t_max and serum concentrations declining at a similar rate.

FIGURE 2.

Geometric mean serum concentrations and biosimilarity, following single 60 mg dose of either RGB‐14‐X or reference denosumab (PK population). AUC_0–last, area under the concentration‐time curve from time 0 to the last quantifiable concentration; AUC_0–inf, AUC from time 0 to infinity; CI, confidence interval; C_max, maximum observed serum concentration; PK, pharmacokinetics. ^aRGB‐14‐X, n = 82; reference denosumab, n = 81. ^bEquivalence threshold: 0.80–1.25.

TABLE 2.

Summary of secondary PK parameters (PK population).

	RGB‐14‐X (N = 83)	Reference denosumab (N = 82)
AUC0–119d, day*μg/mL a	278.7 (22.0)	251.5 (26.7)
AUC119d–last, day*μg/mL a	5.4 (225.1)	3.3 (284.7)
tmax, day, median (range)	10.9 (3.0–27.0)	9.0 (2.0–27.0)
t1/2, day a	6.0 (19.7)	6.0 (27.1)

Note: Data are geometric mean (CV%), unless stated otherwise.

Abbreviations: AUC_0‐119d, area under the concentration‐time curve from time 0 to Day 119; AUC_119d‐last, area under the concentration‐time curve from Day 119 to the last quantifiable concentration; C_max, maximum observed serum concentration; CV, coefficient of variation; PK, pharmacokinetic; t_1/2, terminal elimination half‐life; t_max, time corresponding to occurrence of C_max.

^a RGB‐14‐X n = 82; reference denosumab, n = 81.

3.3. Pharmacodynamics

For both treatments, CTX concentrations showed a biphasic response, rapidly declining after a single 60 mg dose and starting to increase around 146 days post‐dose (Figures 3 and S2). No apparent differences were observed in the profiles for mean concentration and %CfB of CTX (Figures 3 and S2). The maximum reduction (I_max) in CTX concentration was similar for both groups. In addition, the extent and duration in the reduction of CTX levels, as shown by AUEC values, were similar between groups. Taken together, these results suggest the magnitude of the suppressive effect on bone resorption was similar in both groups (Figures 3 and S3).

FIGURE 3.

Arithmetic mean %CfB in serum CTX concentrations and summary statistics, following a single 60 mg dose of either RGB‐14‐X or reference denosumab (PD population). %CfB, percent change from baseline; AUEC, area under the effect‐time curve; CTX, C‐terminal telopeptide of type I collagen; CV, coefficient of variation; I_max, maximum percent inhibition; PD, pharmacodynamics; SD, standard deviation. ^aRGB‐14‐X, n = 79; reference denosumab, n = 80.

3.4. Safety and Immunogenicity

The safety profiles were similar for both groups (Table 3). TEAEs were reported for 62 (74.7%) and 66 (80.5%) participants in the RGB‐14‐X and reference denosumab groups, respectively. No discontinuations or deaths due to TEAEs were reported. Most TEAEs were mild to moderate; three (1.8%) participants reported four events classified as severe TEAEs (nerve compression [n = 1, reference denosumab]; meniscus injury and subsequent arthroscopy [n = 1, reference denosumab]; and influenza [n = 1, RGB‐14‐X]). One participant in each group had a serious TEAE considered not related to study drug (influenza [n = 1, RGB‐14‐X] and nerve compression [n = 1, reference denosumab]). Approximately one‐third of participants experienced TEAEs considered by investigators to be related to study drug (50/165 participants [30%], 85 events); for participants in the RGB‐14‐X and reference denosumab groups, postural dizziness (n = 5 [6.0%] and n = 4 [4.9%]), headache (n = 4 [4.8%] and n = 3 [3.7%]), fatigue (n = 3 [3.6%] and n = 2 [2.4%]) and back pain (n = 3 [3.6%] and n = 0) were the most common related TEAEs, respectively (occurring in ≥ 3% participants in any treatment group; Table S2; Table S3). Injection‐site reactions were reported by two (2.4%) participants in each group. No clinically relevant differences were observed between groups in laboratory test values (including calcium levels), vital signs or ECGs. Despite application of a highly sensitive, drug‐ and target‐tolerant electrochemiluminescent bridging immunogenicity ADA assay [31], no ADAs or NAbs were detected in any of the study samples.

TABLE 3.

Overview of TEAEs, following a single 60 mg dose of either RGB‐14‐X or reference denosumab (safety analysis set).

n (%) [number of events]	RGB‐14‐X (N = 83)	Reference denosumab (N = 82)
Any TEAE	62 (74.7) [211]	66 (80.5) [213]
TEAE related to study drug	25 (30.1%) [45]	25 (30.5%) [40]
TEAE leading to death or study discontinuation	0	0
Any serious TEAE	1 (1.2) [1]	1 (1.2) [1]
Serious TEAE related to study drug	0	0
Injection‐site reaction	2 (2.4) [2]	2 (2.4) [2]
TEAE severity
Mild	30 (36.1) [151]	27 (32.9) [147]
Moderate	31 (37.3) [59]	37 (45.1) [63]
Severe	1 (1.2) [1]	2 (2.4) [3]

Abbreviation: TEAE, treatment‐emergent adverse event.

4. Discussion

Results from this study demonstrate the PK equivalence of a single dose of RGB‐14‐X and reference denosumab in healthy adult males. The primary endpoints were met and the results are in line with the FDA and EMA requirement for demonstrating equivalence of biosimilars with reference therapeutics [25, 26]. Secondary PK outcomes were also similar between groups, as were PD assessments of serum levels of the bone resorption marker CTX. CTX levels dropped rapidly after dosing and began to rise shortly before 6 months, which is consistent with the standard 6‐month dosing interval for denosumab for the treatment of osteoporosis and cancer treatment‐induced bone loss [32, 33, 34, 35].

The safety profiles of RGB‐14‐X and reference denosumab were comparable, with no new or unexpected safety signals reported. Both treatments were similarly well tolerated. The overall prevalence of TEAEs was low in both arms, with numerical differences reflecting natural variation. No ADAs were reported in either treatment arm in this study. In some previous biosimilar studies, higher incidences of ADAs of up to 100% have been reported in both biosimilar and comparator denosumab arms [36, 37]. However, data derived from both the RGB‐14 Phase 1 and Phase 3 studies is consistent with the accumulated experience for the originator products, Xgeva and Prolia, which have shown that denosumab is associated with a very low incidence of ADA formation [13, 14, 38]. One possible explanation for this discrepancy is the potential for false positive results in denosumab ADA assays due to the interference of soluble RANKL [31, 39]. Immunogenicity in this study was assessed using a highly sensitive drug‐ and target‐tolerant ADA assay that included the use of osteoprotegerin to minimize RANKL target interference, as has been previously described [31]. Therefore, this provides solid evidence that RGB‐14‐X will demonstrate a similarly positive benefit/risk profile to originator denosumab, with similarly low immunogenicity [7, 8, 9].

Although RGB‐14‐X is planned to be administered at the higher therapeutic dose of 120 mg Q4W in adults with advanced malignancies involving bone, a lower 60 mg dose was used in this study per FDA and EMA biosimilar guidance to allow the most sensitive comparison of PK between RGB‐14 and reference denosumab. Denosumab is eliminated both by non‐linear target‐mediated drug disposition (which is saturated at therapeutic doses) and linear immunoglobulin G clearance pathways [40]. By utilizing the lowest therapeutic (60 mg) dose level of denosumab in this Phase 1 PK/PD study, both elimination pathways are active, ensuring the most sensitive circumstances to detecting any differences in PK characteristics. Per regulatory guidance demonstrating similarity at the lowest therapeutic dose level for a biosimilar allows extrapolation to the approved higher dose of the originator [25, 26].

As this was a Phase 1 study, the size and heterogeneity of its population were necessarily restricted. Healthy males are considered by the regulatory authorities as more homogeneous with respect to bone health which maximizes the sensitivity to detect differences between RGB‐14‐X and reference denosumab. A parallel‐group design was selected because of the relatively long half‐life (approximately 28 days) of denosumab. A post‐dose follow‐up of 252 days allowed (1) immunogenicity to be investigated and (2) the late‐elimination (non‐linear) phase of denosumab PK and reversible effects on CTX to be adequately characterized, maximizing the sensitivity of immunogenicity and PK/PD assessments to detect differences. Lastly, calcium and vitamin D3 prophylaxis were administered to all participants to prevent hypocalcaemia, as indicated for denosumab and consistent with typical clinical practice.

Biosimilarity of RGB‐14 to the originator denosumab was demonstrated in a robust development program which consisted of structural and functional characterization, a Phase 1 PK/PD in healthy male participants and a Phase III efficacy/safety study in women with postmenopausal osteoporosis (EudraCT: 2020‐006017‐38; ClinicalTrials.gov: NCT05087030) [28]. The equivalent efficacy in the Phase 3 study was assessed by the percentage change from baseline (%CfB) in lumbar spine bone mineral density (BMD) at Week 52: the adjusted mean (95% CI) was 4.89% (3.55, 6.24) for RGB‐14‐P and 4.55% (3.22, 5.87) for denosumab (estimated difference: 0.34; 95% CI: −0.40, 1.09). There were no statistical differences in %CfB in BMD at the total hip and femoral neck. Furthermore, the incidence of vertebral and non‐vertebral fragility fractures was comparable between the two treatment arms [28]. Taken together, the data from both studies suggest that RGB‐14 will effectively replicate the therapeutic benefit of denosumab in a clinical setting, with a similar safety and immunogenicity profile.

The higher cost of denosumab as compared with generically available bisphosphonates may be a barrier to its wider clinical use, despite compelling scientific data underscoring its safety and efficacy for a wide range of conditions [24, 41]. Use of denosumab biosimilars in healthcare systems may improve the accessibility of denosumab and broaden its use for patients where high costs have previously precluded access to the originator treatment [41, 42, 43].

As a treatment with proven equivalence to denosumab, RGB‐14 has the potential to provide a lower‐cost alternative that could improve access to biologic therapies and ultimately improve patient outcomes.

Author Contributions

All authors wrote the manuscript; Z.N., A.K., K.H.‐K., J.K., T.P., and E.J. designed the research; all authors analyzed the data.

Funding

This study was funded by Gedeon Richter (EudraCT: 2020‐003953‐32) and supported by the GINOP‐2.3.4‐15‐2016‐00002 project, which was co‐financed by the European Union and the European Regional Development Fund.

Conflicts of Interest

E.B. reported receiving consulting and/or lecture fees from Gedeon Richter, UCB, and Amgen. J.‐J.B. reported receiving consulting and lecture fees from Gedeon Richter, UCB, and Sandoz. A.S. reported receiving speaker honoraria from Gedeon Richter, Ipsen, J&J, consulting fees from Veracyte, and travel support from J&J. H.S. reported receiving speaker honoraria from Gedeon Richter. Z.N., A.K., K.H.‐K., J.K., T.P., E.J., and F.B.V. are employees of Gedeon Richter. L.S. reported receiving consulting and lecture fees from Gedeon Richter.

Supporting information

Figure S1: Boxplots of serum PK parameters C_max (A), AUC_0–last (B), and AUC_0–inf (C), following a single 60 mg dose either RGB‐14‐X or reference denosumab (PK population).

Figure S2: Arithmetic mean %CfB in serum CTX concentrations Day 1–28 (A) and Day 35–252 (B) following a single 60 mg dose of either RGB‐14‐X or reference denosumab (PD population).

Figure S3: Boxplots of CTX I_max (A) and AUEC (B), following single‐dose RGB‐14‐X and reference denosumab (PD population).

Table S1: Inclusion and exclusion criteria.

Table S2: TEAEs related to study treatment in ≥ 3% of participants in any treatment group, following a single 60 mg dose of either RGB‐14‐X or reference denosumab (safety analysis set).

Table S3: TEAEs in ≥ 3% of participants in any treatment group, following a single 60 mg dose of either RGB‐14‐X or reference denosumab (safety analysis set); Supplementary methods.

Biver E., Body J.‐J., Sachdeva A., et al., “A Randomized Phase 1 Study Comparing the PK, PD, Safety, and Immunogenicity of Proposed Biosimilar RGB‐14‐X and Denosumab in Healthy Adult Males,” Clinical and Translational Science 19, no. 2 (2026): e70468, 10.1111/cts.70468.