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Polyarticular juvenile idiopathic arthritis (pJIA) is a pediatric chronic inflammatory arthritis, much like rheumatoid arthritis (RA) in adults. Drug development for pJIA can potentially be expedited by using extrapolation of efficacy from adult RA; however, the lack of understanding of the response and exposure relationship between pJIA and RA to therapeutic interventions has been a major roadblock. To address this, the objective of our analysis was to conduct a systematic response and exposure comparison between pJIA and RA trials for biologic products. Data from registration RA and pJIA clinical trials (parallel or withdrawal design) for infliximab, tocilizumab, golimumab, and adalimumab were utilized. First, exposure was compared between the pJIA trials and RA pivotal trials. Subsequently, the pJIA vs. RA response similarity was assessed by comparing similar individual subcomponents of the American College of Rheumatology (ACR) scores between the two populations. The exposure comparison demonstrated that at the pJIA trial dose, exposure in pediatric patients was similar to or higher than adults for all biologics evaluated except infliximab, where lower exposure was observed in pJIA patients ≤ 35 kg. Response comparison for individual subcomponents indicated that in a majority of the cases, pJIA response was similar or higher as compared with response from RA trials. Overall, this analysis suggests response similarity between pJIA and RA across the biologic products when exposures are matched between the two populations. These analyses provide support for the use of pharmacokinetic exposure-matching for extrapolation of efficacy from adult RA to pediatric pJIA for the products with established mechanism(s) of action.

Pirfenidone and nintedanib are the first two FDA-approved therapies for treatment of idiopathic pulmonary fibrosis (IPF). The clinical programs for pirfenidone and nintedanib included 1132 patients in the placebo arms and 1691 patients in the treatment arms across 6 trials. We developed a disease progression model to characterize the observed variability in lung function decline, measured as percent predicted forced vital capacity (%p-FVC), and its decrease in decline after treatment. The non-linear longitudinal change in %p-FVC was best described by a Weibull function. The median decreased decline in %p-FVC after treatment was estimated to be 1.50% (95% CI [1.12, 1.79]) and 1.96% (95% CI [1.47, 2.36]) at week 26 and week 52, respectively. Smoking status, weight, %p-FVC, %p-DLco and oxygen use at baseline were identified as significant covariates affecting decline in %p-FVC. The decreased decline in %p-FVC were observed among all subgroups of interest, of which the effects were larger at 1 year compared to 6 months. Based on the disease progression model smoking status and oxygen use at baseline may affect the treatment effect size. At week 52, the decreased decline in %p-FVC for current smokers and patients with oxygen use at baseline were 1.56 (90% CI [1.02, 1.99]) and 2.32 (90% CI [1.74, 2.86]), respectively. These prognostic factors may be used to enrich studies with patients who are more likely to respond to treatment, by demonstrating a lesser decline in lung function, and therefore provide the potential to allow for IPF studies with smaller study populations or shorter durations.

Periodic fever syndromes are a group of rare diseases with a highly variable onset, yet limited treatment options are available for children at an early age. Canakinumab has been approved to treat patients with cryopyrin-associated periodic syndrome, a periodic fever syndrome, and systemic juvenile systemic arthritis, with age cutoffs of 4 years and 2 years, respectively. In 2016, the US Food and Drug Administration (FDA) approved canakinumab, without an age restriction, for the treatment of three conditions of periodic fever syndromes, including familial Mediterranean fever, hyperimmunoglobulin D syndrome/mevalonate kinase deficiency, and tumor necrosis factor receptor-associated periodic syndrome. This review discusses the pharmacokinetic (PK), efficacy, safety, and exposure-response relationship of canakinumab and provides the rationale for dosage recommendation in children younger than 2 years of age with the three conditions of periodic fever syndromes. The approval of canakinumab for these pediatric patients addresses a critical unmet medical need.

Determination of appropriate pharmacokinetic end point to bridge different dosing regimens is often a challenge when developing a new route of administration. Trough concentrations (Ctrough) are often considered the most relevant PK end point to predict efficacy (ACR20/DAS28) in the treatment of rheumatoid arthritis for biologics. However, no systematic research has been conducted to evaluate this approach. We developed a novel strategy to predict the most relevant PK variables that may be used to support a change in the route of administration for biological products. Our analysis indicated that matching only Ctrough when switching from intravenous dosing to subcutaneous dosing with decreasing dosing interval may result in a lower treatment response. If only average concentration (Cave) is considered as the relevant variable, our analysis showed that treatment response may be worsened when switching from subcutaneous dosing to intravenous dosing with increasing dosing interval. The study results demonstrated that matching a single pharmacokinetic end point (Ctrough or Cave) may not be sufficient to ensure efficacy when switching between intravenous dosing and subcutaneous dosing. A practical novel pharmacokinetic bridging approach is provided to support a change in the route of administration for biological products.

The aim of the study was to evaluate the quantitative relationship between duration of severe neutropenia (DSN, the efficacy endpoint) and area under effect curve of absolute neutrophil counts (ANC-AUEC, the pharmacodynamic endpoint), based on data from filgrastim products, a human granulocyte colony-stimulating factor (G-CSF). Clinical data from filgrastim product comparator and test arms of two randomized, parallel-group, phase III studies in breast cancer patients treated with myelosuppressive chemotherapy were utilized. A zero-inflated Poisson regression model best described the negative correlation between DSN and ANC-AUEC. The models predicted that with 10 × 109 day/L of increase in ANC-AUEC, the mean DSN would decrease from 1.1 days to 0.93 day in Trial 1 and from 1.2 days to 1.0 day in Trial 2. The findings of the analysis provide useful information regarding the relationship between ANC and DSN that can be used for dose selection and optimization of clinical trial design for G-CSF.

On August 26, 2011, the U.S. Food and Drug Administration (FDA) approved crizotinib (XALKORI Capsules, Pfizer Inc.) for treatment of patients with locally advanced or metastatic non–small cell lung cancer (NSCLC) that is anaplastic lymphoma kinase (ALK) positive as detected by an FDA-approved test. The Vysis ALK Break-Apart FISH Probe Kit (Abbott Molecular, Inc.) was approved concurrently. In two multicenter, single-arm trials, patients with locally advanced or metastatic ALK-positive NSCLC previously treated with one or more systemic therapies received crizotinib orally at a dose of 250 mg twice daily. In 119 patients with ALK-positive NSCLC by local trial assay, the objective response rate (ORR) was 61% [95% confidence intervals (CI), 52%–70%] with a median response duration of 48 weeks. In 136 patients with ALK-positive NSCLC by the to-be-marketed test, the ORR was 50% (95% CI, 42%–59%) with a median response duration of 42 weeks. The most common adverse reactions (≥25%) were vision disorder, nausea, diarrhea, vomiting, edema, and constipation. Accelerated approval was granted on the basis of the high ORRs and durable responses. On November 20, 2013, crizotinib received full approval based on an improvement in progression-free survival in patients with metastatic ALK-positive NSCLC previously treated with one platinum-based chemotherapy regimen. Clin Cancer Res; 20(8); 2029–34. ©2014 AACR.

On December 11, 2015, the FDA approved uridine triacetate (VISTOGARD; Wellstat Therapeutics Corporation) for the emergency treatment of adult and pediatric patients following a fluorouracil or capecitabine overdose regardless of the presence of symptoms, and of those who exhibit early-onset, severe, or life-threatening toxicity affecting the cardiac or central nervous system, and/or early onset, unusually severe adverse reactions (e.g., gastrointestinal toxicity and/or neutropenia) within 96 hours following the end of fluorouracil or capecitabine administration. Uridine triacetate is not recommended for the nonemergent treatment of adverse reactions associated with fluorouracil or capecitabine because it may diminish the efficacy of these drugs, and the safety and efficacy of uridine triacetate initiated more than 96 hours following the end of administration of these drugs has not been established. The approval is based on data from two single-arm, open-label, expanded-access trials in 135 patients receiving uridine triacetate (10 g or 6.2 g/m2 orally every 6 hours for 20 doses) for fluorouracil or capecitabine overdose, or who exhibited severe or life-threatening toxicities within 96 hours following the end of fluorouracil or capecitabine administration. Ninety-six percent of patients met the major efficacy outcome measure, which was survival at 30 days or survival until the resumption of chemotherapy, if prior to 30 days. The most common adverse reactions were vomiting, nausea, and diarrhea. This article summarizes the FDA review of this New Drug Application, the data supporting approval of uridine triacetate, and the unique regulatory situations encountered by this approval. Clin Cancer Res; 22(18); 4545–49. ©2016 AACR.

To characterize the pharmacokinetic (PK) and pharmacodynamic (PD) properties of a monoclonal antibody directed against the B-cell activating factor (BAFF) receptor 3 (BR3), following intravenous (IV) and subcutaneous (SC) administration in mice.Single IV doses of 0.2, 2.0 and 20 mg/kg and a single SC injection of 20 mg/kg of anti-BR3 antibody was administered to mice. Serum drug and BAFF concentrations and splenic B-cell concentrations were measured at various time points. Pooled PK profiles were described by a two-compartmental model with time-dependent nonlinear elimination, and BAFF profiles were defined by an indirect response model. Fractional receptor occupancy served as the driving function for a competitive reversible antagonism model to characterize B-cell dynamics.Noncompartmental analysis revealed a decrease in drug clearance (31.3 to 7.93 mL/day/kg) with increasing IV doses. The SC dose exhibited slow absorption (T(max) = 2 days) and complete bioavailability. All doses resulted in a dose-dependent increase in BAFF concentrations and decrease in B-cell counts. The proposed model reasonably captured complex PK/PD profiles of anti-BR3 antibody after IV and SC administration.A mechanistic model was developed that describes the reversible competition between anti-BR3 antibody and BAFF for BR3 receptors and its influence on B-cell pharmacodynamics.

On April 6, 2011, the U.S. Food and Drug Administration approved vandetanib (Caprelsa tablets; AstraZeneca Pharmaceuticals LP) for the treatment of symptomatic or progressive medullary thyroid cancer in patients with unresectable, locally advanced, or metastatic disease. Vandetanib is the first drug approved for this indication, and this article focuses on the basis of approval. Approval was based on the results of a double-blind trial conducted in patients with medullary thyroid carcinoma. Patients were randomized 2:1 to vandetanib, 300 mg/d orally (n = 231), or to placebo (n = 100). The primary objective was demonstration of improvement in progression-free survival (PFS) with vandetanib compared with placebo. Other endpoints included evaluation of overall survival and objective response rate. The PFS analysis showed a marked improvement for patients randomized to vandetanib (hazard ratio = 0.35; 95% confidence interval, 0.24–0.53; P < 0.0001). The objective response rate for the vandetanib arm was 44% compared with 1% for the placebo arm. The most common grade 3 and 4 toxicities (>5%) were diarrhea and/or colitis, hypertension and hypertensive crisis, fatigue, hypocalcemia, rash, and corrected QT interval (QTc) prolongation. This approval was based on a statistically significant and clinically meaningful improvement in PFS. Given the toxicity profile, which includes prolongation of the QT interval and sudden death, only prescribers and pharmacies certified through the vandetanib Risk Evaluation Mitigation Strategy Program are able to prescribe and dispense vandetanib. Treatment-related risks should be taken into account when considering the use of vandetanib in patients with indolent, asymptomatic, or slowly progressing disease. Clin Cancer Res; 18(14); 3722–30. ©2012 AACR.

This work describes the considerations that led to the approval by the U.S. Food and Drug Administration (FDA), on November 15, 2010, of eribulin mesylate (Halaven; Eisai, Inc.) for the treatment of patients with refractory metastatic breast cancer. The FDA review focused primarily on the results of a single randomized, open-label, multicenter trial of 762 patients with refractory locally advanced or metastatic breast cancer. The patients were randomized to receive eribulin or any single-agent treatment of the physician's choice, selected prior to randomization. The FDA's approval of eribulin mesylate was based on demonstration of a statistically significant prolongation of overall survival (OS) in patients who had been randomized to receive eribulin. The median OS was 13.1 months in the eribulin arm compared with 10.6 months in the control arm [HR 0.81 (95% CI, 0.66–0.99); P = 0.041]. Treatment with eribulin did not show a statistically significant treatment effect [HR 0.87 (95% CI, 0.71–1.05)] on progression-free survival as determined by independent review. This approval highlights the appropriate use of an innovative trial design and shows that improvement in OS is an achievable endpoint in the setting of advanced breast cancer. On the basis of the different conclusions arising from the OS and progression-free survival results, investigators should consider using OS as a primary endpoint in clinical trials for refractory breast cancer. Clin Cancer Res; 18(6); 1496–505. ©2012 AACR.

This study aims to characterize the pharmacodynamic properties of denosumab, a RANK ligand inhibitor, and ibandronate, a bisphosphonate, using an integrated bone homeostasis model in postmenopausal women. Mean temporal profiles of denosumab, serum and urine N-telopeptide (sNTX, uNTX), lumbar spine bone mineral density (BMD) following denosumab administration, and urine C-telopeptide (uCTX) and lumbar spine BMD upon ibandronate administration were extracted from the literature. A mechanistic model was developed that integrates denosumab pharmacokinetics with binding to RANK ligand and ibandronate inhibition of osteoclast precursor differentiation to active osteoclasts (AOC). Biomarker concentrations were linked to the AOC pool. The BMD was characterized by a turnover model with stimulation of bone formation and degradation by AOB (active osteoblasts) and AOC pools. The estimated basal sNTX, uNTX and uCTX concentrations were 7.24 nm, 14.4 nmol/mmolCr and 31µg/mmolCr. The BMD degradation rate was 0.00161 day(-1) with stimulation constants associated with AOB and AOC of 1214 and 790 pm(-1) . The plasma ibandronate concentration producing 50% of maximum inhibition of osteoclast differentiation was 522 ng/l. The integrated model, which incorporates multiple pathways of therapeutic intervention, quantitatively describes changes in clinical biomarkers of bone turnover and BMD after denosumab and ibandronate exposures in postmenopausal women.

The purpose of this study was to examine the role of dose selection on population pharmacokinetic (PK) parameter estimation using a rapid binding approximation of a target-mediated drug disposition (TMDD) model previously developed for interferon-β (IFN-β). A total of 50 replicate datasets each containing 100 subjects were created using NONMEM. The study design included IV injection of IFN-β followed by the SC route in a crossover manner, with each dose and route of administration separated by a 1,000 h washout period. Serial plasma PK samples were simulated up to 48 h for all subjects following each dose. Population mean PK parameters were re-estimated in NONMEM for each simulated dataset using the same TMDD model after including the following doses (MIU/kg): (A) 1, 3 and 10 (original study); (B) 1, 3 and 7; (C) 1, 3 and 5; (D) 1, 3 and 4; (E) 1 and 3; (F) 3 and 10; or (G) 10 MIU/kg only. Bias in the model fit was assessed by calculating the percent prediction error (PE%) for each of the population mean PK parameters relative to the estimates obtained from the fit to the 1, 3, and 10 MIU/kg doses (Case A). Relatively unbiased population mean PK parameter estimates (median PE%8%) were obtained only when the study design included 1, 3 and a minimum higher dose of 7 MIU/kg. Bias increased for various parameters when the highest dose was less than 7 MIU/kg along with 1 and 3 MIU/kg being the low and intermediate dose levels. An increase in the bias for binding capacity, R(tot), and the equilibrium dissociation constant, K (D), was observed as the highest dose included in the dataset was reduced from 5 to 3 MIU/kg (median PE% ranged from -4.71 to -23.9% and -4.76 to -34.6%). Similar increases in the range of median PE% were also observed for other model parameters as the highest dose was reduced from 5 to 3 MIU/kg. Severely biased results were obtained from the study design that included only the 10 MIU/kg dose (Case G) suggesting that it is not sufficient to study just a high dose group. This bias was greatly reduced (median PE%14%) for all parameters except K (D) when the 3 and 10 MIU/kg doses were co-modeled (Case F). Plots of the PE% for R(tot) and K (D) versus the molar ratio of maximum dose to R(tot) suggest that study designs should evaluate at least one IFN-β dose 3.5- to 4-fold higher than R(tot) along with the 1 and 3 MIU/kg dose levels to obtain unbiased population PK parameter estimates. In summary, for the IFN-β model and study design, dose selection influences the ability to generate relatively unbiased population mean TMDD parameter estimates, which is based on maximum dose levels relative to R(tot). This simulation study highlights the role of dose selection in optimal study design strategies for drugs such as IFN-β that exhibit TMDD properties.

Purpose: On September 24, 2009, the U.S. Food and Drug Administration granted accelerated approval for Folotyn (pralatrexate injection, Allos Therapeutics, Inc.) as a single agent for the treatment of patients with relapsed or refractory peripheral T-cell lymphoma (PTCL); it is the first drug approved for this indication. Experimental Design: This review was based on study PDX-008, a phase II, single-arm, nonrandomized, open-label, international, multicenter trial, designed to evaluate the safety and efficacy of pralatrexate when administered concurrently with vitamin B12 and folic acid supplementation in patients with relapsed or refractory PTCL. Results: The overall response rate was 27% in 109 evaluable patients [95% confidence interval (CI), 19–36%]. Twelve percent of 109 evaluable patients (95% CI, 7–20%)] had a response duration of ≥14 weeks. Six of these 13 patients achieved a complete response, and one patient had complete response unconfirmed. The most common grade 3 and 4 toxicities were thrombocytopenia, mucositis, and neutropenia. Conclusion: This accelerated approval was based on a response rate that is reasonably likely to predict clinical benefit in this heavily pretreated patient population with this rare disease. The applicant has committed to conducting postmarketing clinical trials to assess clinical benefit. The recommended starting dose of pralatrexate in patients with relapsed or refractory PTCL is 30 mg/m2 via intravenous push over 3 to 5 min weekly for 6 weeks followed by a one-week rest (one cycle). Intramuscular injection of 1 mg vitamin B12 should be administered every 8 to 10 weeks along with 1.0 mg folic acid given orally once a day. Clin Cancer Res; 16(20); 4921–7. ©2010 AACR.

To date, Exposure-Response (E-R) analyses have been recognized and routinely utilized in regulatory reviews, mainly to address key questions such as whether the proposed dosing regimen for a new drug is optimal or is warranted for further adjustment in specific populations in the context of the overall risk/benefit profile. This chapter will start from a summary of E-R methods commonly used in current applications followed with new methodology development and applications of E-R analyses on other aspects of reviews. Reporting on new methodology development is focused on case–control analyses when drug exposure is confounded with baseline disease status for several antibody oncology drugs. Reporting on applications of E-R analysis in new areas of review falls into using E-R analysis to derive the effect size for the noninferiority trial and subgroup analyses to identify favorable risk/benefit profile in specific population(s). The chapter also mentioned the potential role of E-R analysis in precision medicine by leveraging individual drug exposure to achieve balanced risk/benefit at the individual level.

The FDA approved lenvatinib (Lenvima, Eisai Inc.) for the treatment of patients with locally recurrent or metastatic, progressive, radioactive iodine-refractory (RAI-refractory) differentiated thyroid cancer (DTC). In an international, multicenter, double-blinded, placebo-controlled trial (E7080-G000-303), 392 patients with locally recurrent or metastatic RAI-refractory DTC and radiographic evidence of disease progression within 12 months prior to randomization were randomly allocated (2:1) to receive either lenvatinib 24 mg orally per day (n = 261) or matching placebo (n = 131) with the option for patients on the placebo arm to receive lenvatinib following independent radiologic confirmation of disease progression. A statistically significant prolongation of progression-free survival (PFS) as determined by independent radiology review was demonstrated [HR, 0.21; 95% confidence interval (CI), 0.16–0.28; P < 0.001, stratified log-rank test], with an estimated median PFS of 18.3 months (95% CI, 15.1, NR) in the lenvatinib arm and 3.6 months (95% CI, 2.2–3.7) in the placebo arm. The most common adverse reactions, in order of decreasing frequency, observed in the lenvatinib-treated patients were hypertension, fatigue, diarrhea, arthralgia/myalgia, decreased appetite, decreased weight, nausea, stomatitis, headache, vomiting, proteinuria, palmar-plantar erythrodysesthesia syndrome, abdominal pain, and dysphonia. Adverse reactions led to dose reductions in 68% of patients receiving lenvatinib at the 24 mg dose and 18% of patients discontinued lenvatinib for adverse reactions leading to residual uncertainty regarding the optimal dose of lenvatinib. Clin Cancer Res; 21(23); 5205–8. ©2015 AACR.

Canagliflozin (INVOKANA™) is approved as an adjunct to diet and exercise to improve glycemic control in adults with type 2 diabetes mellitus (T2DM). Canagliflozin inhibits renal sodium-glucose co-transporter 2 (SGLT2), thereby, reducing reabsorption of filtered glucose and increasing urinary glucose excretion. Given the mechanism of action of SGLT2 inhibitors, we assessed the interplay between renal function, efficacy (HbA1c reduction), and safety (renal adverse reactions). The focus of this article is to highlight the FDA's quantitative clinical pharmacology analyses that were conducted to support the regulatory decision on dosing in patients with renal impairment (RI). The metrics for assessment of efficacy for T2DM drugs is standard; however, there is no standard method for evaluation of renal effects for diabetes drugs. Therefore, several analyses were conducted to assess the impact of canagliflozin on renal function (as measured by eGFR) based on available data. These analyses provided support for approval of canagliflozin in T2DM patients with baseline eGFR ≥ 45 mL/min/1.73 m(2) , highlighting a data-driven approach to dose optimization. The availability of a relatively rich safety dataset (ie, frequent and early measurements of laboratory markers) in the canagliflozin clinical development program enabled adequate assessment of benefit-risk balance in various patient subgroups based on renal function.

Pharmacometric analyses have become an increasingly important component of New Drug Application (NDA) and Biological License Application (BLA) submissions to the US FDA to support drug approval, labelling and trial design decisions. Pharmacometrics is defined as a science that quantifies drug, disease and trial information to aid drug development, therapeutic decisions and/or regulatory decisions. In this report, we present the results of a survey evaluating the impact of pharmacometric analyses on regulatory decisions for 198 submissions during the period from 2000 to 2008. Pharmacometric review of NDAs included independent, quantitative analyses by FDA pharmacometricians, even when such analysis was not conducted by the sponsor, as well as evaluation of the sponsor’s report. During 2000–2008, the number of reviews with pharmacometric analyses increased dramatically and the number of reviews with an impact on approval and labelling also increased in a similar fashion. We also present the impact of pharmacometric analyses on selection of paediatric dosing regimens, approval of regimens that had not been directly studied in clinical trials and provision of evidence of effectiveness to support a single pivotal trial. Case studies are presented to better illustrate the role of pharmacometric analyses in regulatory decision making.

This study aims to characterize the pharmacodynamic properties of denosumab, a RANK ligand inhibitor, and ibandronate, a bisphosphonate, using an integrated bone homeostasis model in postmenopausal women. Mean temporal profiles of denosumab, serum and urine N-telopeptide (sNTX, uNTX), lumbar spine bone mineral density (BMD) following denosumab administration, and urine C-telopeptide (uCTX) and lumbar spine BMD upon ibandronate administration were extracted from the literature. A mechanistic model was developed that integrates denosumab pharmacokinetics with binding to RANK ligand and ibandronate inhibition of osteoclast precursor differentiation to active osteoclasts (AOC). Biomarker concentrations were linked to the AOC pool. The BMD was characterized by a turnover model with stimulation of bone formation and degradation by AOB (active osteoblasts) and AOC pools. The estimated basal sNTX, uNTX and uCTX concentrations were 7.24 nm, 14.4 nmol/mmolCr and 31µg/mmolCr. The BMD degradation rate was 0.00161 day(-1) with stimulation constants associated with AOB and AOC of 1214 and 790 pm(-1) . The plasma ibandronate concentration producing 50% of maximum inhibition of osteoclast differentiation was 522 ng/l. The integrated model, which incorporates multiple pathways of therapeutic intervention, quantitatively describes changes in clinical biomarkers of bone turnover and BMD after denosumab and ibandronate exposures in postmenopausal women.

Target mediated drug disposition (TMDD) describes the phenomenon where high affinity binding of a drug to its pharmacological target (enzymes or receptors) significantly alters the pharmacokinetic profile of the drug. A rapid binding model replaces the often inestimable binding micro-constants (k (on) and k (off)) of TMDD models with the equilibrium dissociation constant (K (D)) by assuming rapid binding of the drug to its target. The purpose of this study is to examine the validity of the rapid binding assumption and the pharmacokinetic properties of this model. Temporal profiles of free drug in plasma and a non-specific distribution site, free receptor, and the pharmacodynamically relevant, drug-receptor complex obtained from the rapid binding model compared favorably with the full TMDD model for small values of the parameter epsilon, which represents the ratio of the time required for drug-receptor binding relative to the time required for drug to be cleared from the system. The effect of escalating drug doses on the temporal characteristics and the comparison between the two models has been numerically investigated. A closer match between the full and rapid binding models is observed for high doses. Analysis for very large doses (Dose/V (c)) relative to endogenous steady-state receptor concentration (R (ss)), reveals that the rapid binding model reduces to a standard two compartmental model with a plasma compartment with linear drug elimination and a peripheral compartment. Decreasing clearance with increasing dose and decreasing R (ss) indicates that for drugs exhibiting TMDD, the relative ratio of R (ss) and dose is an important determinant of the pharmacokinetic properties rather than the individual parameters alone. An analytical solution derived for clearance shows that the primary elements of the apparent clearance of the drug are the linear clearance given by k (el) V (c), the non-linear clearance due to drug-receptor complex internalization (k (int)), and the ratio of AUC values of the receptor complex to that of free drug. Overall, simulations and analytical techniques applied here provide a better understanding of the validity of the rapid binding model and provide guidelines for its application.

The purpose of this study is to couple a cellular bone homeostasis model with the pharmacokinetics (PK) and mechanism of action of denosumab, an inhibitor of receptor activator of nuclear factor-kappaB ligand, to characterize the time course of serum N-telopeptide (NTX), a bone resorption biomarker, following single escalating doses in multiple myeloma (MM) patients. Mean PK and median serum NTX temporal profiles were extracted from a previously conducted randomized, double-blind, double-dummy, active-controlled, multicenter study including 25 MM patients receiving escalating denosumab doses. Nonlinear denosumab PK profiles were well described by a target-mediated disposition model that includes rapid binding of drug to its pharmacological target. Fixed PK profiles were integrated into a previously reported theoretical cellular model of osteoblast-osteoclast interactions, and the NTX concentrations were linked to a resorbing active osteoclast (AOC) pool by a nonlinear transfer function. Reasonable fits were obtained for the NTX profiles from maximal likelihood estimation using the final model. Transfer function parameters, including the basal NTX level and the AOC concentration producing 50% of maximal NTX production, were estimated with good precision as 5.55 nM and 1.88 x 10(-5) pM. An indirect response model for inhibition of NTX production by denosumab was also used to characterize the data. Although this model adequately characterized the pharmacodynamic data, simulations conducted with the full model reveal that a cellular model coupled with clinical data has the distinct advantage of not only quantitatively describing data but also providing new testable hypotheses on the role of cellular system variables on drug response.