Long-term safety and efficacy of ozanimod in relapsing multiple sclerosis: Up to 5 years of follow-up in the DAYBREAK open-label extension trial.

BACKGROUND: Ozanimod, an oral sphingosine 1-phosphate receptor 1 and 5 modulator, is approved in multiple countries for treatment of relapsing forms of MS. OBJECTIVE: To characterize long-term safety and efficacy of ozanimod. METHODS: Patients with relapsing MS who completed a phase 1‒3 ozanimod trial were eligible for an open-label extension study (DAYBREAK) of ozanimod 0.92 mg/d. DAYBREAK began 16 October 2015; cutoff for this interim analysis was 2 February 2021. RESULTS: This analysis included 2494 participants with mean 46.8 (SD 11.9; range 0.033‒62.7) months of ozanimod exposure in DAYBREAK. During DAYBREAK, 2143 patients (85.9%) had treatment-emergent adverse events (TEAEs; similar in nature to those in the parent trials), 298 (11.9%) had a serious TEAE, and 75 (3.0%) discontinued treatment due to TEAEs. Serious infections (2.8%), herpes zoster infections (1.7%), confirmed macular edema cases (0.2%), and cardiac TEAEs (2.8%) were infrequent. Adjusted annualized relapse rate was 0.103 (95% confidence interval, 0.086‒0.123). Over 48 months, 71% of patients remained relapse free. Adjusted mean numbers of new/enlarging T2 lesions/scan and gadolinium-enhancing lesions were low and similar across parent trial treatment subgroups. CONCLUSIONS: This long-term extension of ozanimod trials confirmed a favorable safety/tolerability profile and sustained benefit on clinical and magnetic resonance imaging measures of disease activity.

Ozanimod in relapsing multiple sclerosis: Pooled safety results from the clinical development program.

BACKGROUND: Ozanimod, an oral sphingosine 1-phosphate receptor 1 and 5 modulator, is approved in multiple countries for the treatment of relapsing multiple sclerosis (RMS). In phase 3 trials, ozanimod was well tolerated and superior to interferon beta-1a 30 µg once-weekly in reducing clinical and radiologic disease activity. The objective of this integrated safety analysis was to evaluate the safety of extended ozanimod exposure in participants with RMS from all clinical trials and compare it with phase 3 trial data. METHODS: We report pooled incidence and study duration‒adjusted incidence rates (IR) of treatment-emergent adverse events (TEAEs) from an interim data cut (January 31, 2019) of RMS participants treated with ozanimod. Data were pooled from a phase 1 pharmacokinetic/pharmacodynamic trial, a placebo-controlled phase 2 trial with dose-blinded extension, 2 large active-controlled phase 3 trials, and an open-label extension (OLE). Results were compared with pooled phase 3 trial data. RESULTS: At the data cutoff, 2631 RMS participants had exposure to ozanimod 0.92 mg (mean 32.0 months) and 2787 had exposure to either ozanimod 0.46 or 0.92 mg (mean 37.1 months). The IRs per 1000 person-years (PY) for any TEAE (772.2) and serious TEAEs (33.2) in the overall population were similar to those in the phase 3 population (896.1 and 31.2, respectively). There were no serious opportunistic infections. There were no second-degree or higher atrioventricular blocks on electrocardiogram. Hepatic enzyme elevations declined over time. Malignancy rates remained low with longer exposure. Pulmonary function tests showed minimal reductions in lung function. Seven ozanimod-treated participants with comorbid risk factors had confirmed macular edema, including 3 in the ongoing OLE. CONCLUSIONS: Safety results in this larger RMS population with greater ozanimod exposure demonstrated no new safety concerns and were consistent with phase 3 trial results.

Long-term Efficacy and Tolerability of RPC4046 in an Open-Label Extension Trial of Patients With Eosinophilic Esophagitis.

BACKGROUND & AIMS: The short-term efficacy of RPC4046, a monoclonal antibody against interleukin-13, has been shown in patients with eosinophilic esophagitis (EoE). We investigated the long-term efficacy and safety of RPC4046 in an open-label, long-term extension (LTE) study in adults with EoE. METHODS: We analyzed data from 66 patients who completed the 16-week, double-blind, induction portion of a phase 2 study of RPC4046 (180 mg or 360 mg/wk) vs placebo and then completed a 52-week LTE, receiving open-label RPC4046 360 mg/wk. The study was conducted at 28 centers in 3 countries; patients were enrolled between September 2014 and January 2017. Outcomes were stratified by double-blind dose group and included esophageal eosinophil counts, EoE endoscopic reference score, EoE histologic scoring system score, symptom-based EoE activity index score, and safety. RESULTS: By week 12 of the LTE, esophageal eosinophil mean and peak counts, total EoE endoscopic reference scores, and EoE histologic scoring system grade and stage scores did not differ considerably between patients who originally received placebo vs RPC4046. Most patients maintained responses through week 52. Symptom remission (symptom-based EoE activity index score, ≤20) increased from 14% at LTE entry to 67% at LTE week 52 in placebo‒RPC4046 patients and from 30% to 54% in RPC4046‒RPC4046 (either dose) patients. Of the 28 patients who did not have a histologic response to RPC4046 during the double-blind induction phase, 10 patients (36%) achieved response during the LTE. The most common adverse events were upper respiratory tract infection (21%) and nasopharyngitis (14%). CONCLUSIONS: One year of treatment with RPC4046 is generally well tolerated and results in continued improvement and/or maintenance of endoscopic, histologic, and clinical measures of EoE disease activity relative to baseline. TRIAL REGISTRATION: NCT02098473.

Non-infectious comorbidity in patients with multiple sclerosis: A national cohort study in Sweden.

BACKGROUND: Comorbidity is of significant concern in multiple sclerosis (MS). Few population-based studies have reported conditions occurring in MS after diagnosis, especially in contemporary cohorts. OBJECTIVE: To explore incident comorbidity, mortality and hospitalizations in MS, stratified by age and sex. METHODS: In a Swedish population-based cohort study 6602 incident MS patients (aged ≥18 years) and 61,828 matched MS-free individuals were identified between 1 January 2008 and 31 December 2016, using national registers. Incidence rates (IRs) and incidence rate ratios (IRRs) with 95% CI were calculated for each outcome. RESULTS: IRs of cardiovascular disease (CVD) were higher among MS patients than MS-free individuals, (major adverse CVD: IRR 1.42; 95% CI 1.12-1.82; hemorrhagic/ischemic stroke: 1.46; 1.05-2.02; transient ischemic attack: 1.65; 1.09-2.50; heart failure: 1.55; 1.15-2.10); venous thromboembolism: 1.42; 1.14-1.77). MS patients also had higher risks of several non-CVDs such as autoimmune conditions (IRR 3.83; 3.01-4.87), bowel dysfunction (2.16; 1.86-2.50), depression (2.38; 2.11-2.68), and fractures (1.32; 1.19-1.47), as well as being hospitalized and to suffer from CVD-related deaths ((1.91; 1.00-3.65), particularly in females (3.57; 1.58-8.06)). CONCLUSION: MS-patients experience a notable comorbidity burden which emphasizes the need for integrated disease management in order to improve patient care and long-term outcomes of MS.

Infections in patients with multiple sclerosis: A national cohort study in Sweden.

BACKGROUND: Multiple sclerosis (MS) patients have an increased risk of infections, but few population-based studies have reported infections occurring in MS in the years immediately after diagnosis. OBJECTIVE: To explore incident infections in MS, stratified by age and sex. METHODS: In a Swedish population-based cohort study 6602 incident MS patients (aged ≥18 years), matched at diagnosis with 61,828 matched MS-free individuals were identified between 1st January 2008 and 31st December 2016, using national registers. Incidence rates (IR) and incidence rate ratios (IRR) with 95% CI were calculated for each outcome. RESULTS: The IRRs were 2.54 (95% CI 2.28-2.83) for first serious infection and 1.61 (1.52-1.71) for first non-serious infection. Compared with MS-free individuals, MS patients had higher IRs for skin, respiratory/throat infections, pneumonia/influenza, bacterial, viral, and fungal infections, with the highest IRR observed for urinary tract/kidney infections (2.44; 2.24-2.66). The cumulative incidence for most of these infections was higher among MS patients than MS-free individuals, both 0 to <5 and 5 to <9 years after index date. CONCLUSION: The burden of infections around the time of MS diagnosis and subsequent infection risk, underscore the need for careful considerations regarding the risk-benefit across different disease-modifying therapies.

Ozanimod induction therapy for patients with moderate to severe Crohn's disease: a single-arm, phase 2, prospective observer-blinded endpoint study.

BACKGROUND: Although treatment of Crohn's disease has improved with development of tumour necrosis factor antagonists, fewer than 50% of patients have sustained benefit. Durable maintenance therapy with orally administered alternative treatments remains an unmet need. We aimed to evaluate the effects of ozanimod, an oral agent selectively targeting sphingosine-1-phosphate receptor subtypes 1 and 5, on endoscopic disease activity in Crohn's disease. METHODS: STEPSTONE was a phase 2, uncontrolled, multicentre trial in adults with moderately to severely active Crohn's disease recruited at 28 hospital and community research centres in Canada, the USA, Hungary, Poland, and Ukraine. All patients began treatment with a 7-day dose escalation (4 days on ozanimod 0·25 mg daily followed by 3 days at 0·5 mg daily). Patients then received ozanimod 1·0 mg oral capsule daily for a further 11 weeks, for a 12-week induction period, followed by a 100-week extension. The primary endpoint was change in Simple Endoscopic Score for Crohn's Disease (SES-CD) from baseline to week 12, as determined by a blinded central reader. Data are reported for the intention-to-treat population. This trial is registered with ClinicalTrials.gov, number NCT02531113 and EudraCT, number 2015-002025-19, and is completed. FINDINGS: 69 patients were enrolled between Nov 17, 2015, and Aug 18, 2016. At week 12, the mean change from baseline in SES-CD was -2·2 (SD 6·0); 16 (23·2%, 95% CI 13·9-34·9) patients experienced endoscopic response. A reduction from baseline in Crohn's Disease Activity Index (CDAI) score also was observed (mean change -130·4 [SD 103·9]). Clinical remission (CDAI <150 points) was shown in 27 (39·1%, 95% CI 27·6-51·6) patients and response (CDAI decrease from baseline ≥100) in 39 (56·5%, 95% CI 44·0-68·4) of patients. The mean change from baseline in two-item patient-reported outcome (PRO2, stool frequency, abdominal pain scores) score was -66·1 (SD 65·4). Mean change from baseline in Geboes Histology Activity Score (GHAS) was -5·9 (SD 11·0) and in Robart's Histopathology Index (RHI) -10·6 (25·1). Adverse events were most frequently those attributed to Crohn's disease, most commonly Crohn's disease (flare) in 18 (26%) patients. The most commonly reported serious treatment-related adverse events were Crohn's disease (six [9%]) and abdominal abscess (two [3%]). INTERPRETATION: Endoscopic, histological, and clinical improvements were seen within 12 weeks of initiating ozanimod therapy in patients with moderately to severely active Crohn's disease. Phase 3 placebo-controlled trials have been initiated. FUNDING: Celgene Corporation.

Incident cardiovascular disease in patients diagnosed with multiple sclerosis: A multi-database study.

BACKGROUND: Recent data on rates of cardiovascular disease (CVD) in patients after MS diagnosis are sparse. OBJECTIVE: To describe incident CVD in MS patients after diagnosis compared with a matched non-MS population. METHODS: We conducted a matched cohort study in two separate electronic medical databases, the United States Department of Defense military health care system and the United Kingdom's Clinical Practice Research Datalink GOLD. The study population included all patients with a first recorded diagnosis of MS and no history of CVD or selected measurable comorbidities associated with CVD and matched non-MS patients who were also free of CVD and the CVD associated comorbidities. We identified incident CVD outcomes first recorded after the MS diagnosis / matched date and calculated incidence rates and incidence rate ratios by type of CVD. RESULTS: Rates of venous thromboembolism and peripheral vascular disease were 2-fold higher among MS than non-MS patients in both databases and the risk of myocardial infarction was 2.5 times higher among female MS patients compared with non-MS females in both databases. Other CVD outcomes were not consistent between databases. CONCLUSION: MS patients in the UK and the US have increased risk of venous thromboembolism and peripheral vascular disease. The risk of myocardial infarction is increased among female MS patients.

Safety and efficacy of ozanimod versus interferon beta-1a in relapsing multiple sclerosis (RADIANCE): a multicentre, randomised, 24-month, phase 3 trial.

BACKGROUND: Ozanimod is a sphingosine 1-phosphate receptor modulator, which selectively binds to sphingosine 1-phosphate receptor subtypes 1 and 5 with high affinity. In the RADIANCE phase 2 study in participants with relapsing multiple sclerosis, ozanimod was associated with better efficacy than placebo on MRI measures and was well tolerated. The RADIANCE phase 3 study aimed to confirm the safety and efficacy of ozanimod versus interferon beta-1a in individuals with relapsing multiple sclerosis. METHODS: We did a 24-month, multicentre, double-blind, double-dummy phase 3 trial in participants with relapsing multiple sclerosis at 147 medical centres and clinical practices in 21 countries. Participants were aged 18-55 years, had multiple sclerosis according to 2010 McDonald criteria, a relapsing clinical course, brain MRI lesions consistent with multiple sclerosis, an expanded disability status scale score of 0·0-5·0, and either at least one relapse within 12 months before screening or at least one relapse within 24 months before screening plus at least one gadolinium-enhancing lesion within the 12 months before randomisation. Participants were randomly assigned (1:1:1) via an interactive voice response system to daily oral ozanimod 1·0 mg or 0·5 mg or weekly intramuscular interferon beta-1a 30 µg. Participants, investigators, and study staff were masked to treatment allocation. The primary endpoint was annualised relapse rate (ARR) over 24 months. The primary analysis was done in the intention-to-treat population of all participants who received study drug and safety was assessed in all randomly assigned participants who received study drug, grouped by highest dose of ozanimod received. This trial is registered at ClinicalTrials.gov, NCT02047734, and EudraCT, 2012-002714-40. FINDINGS: Between Dec 27, 2013, and March 31, 2015, we screened 1695 participants, of which 375 did not meet inclusion criteria. 1320 participants were enrolled and randomly assigned to a group, of whom 1313 received study drug (433 assigned to ozanimod 1·0 mg, 439 assigned to ozanimod 0·5 mg, and 441 assigned to interferon beta-1a) and 1138 (86·7%) completed 24 months of treatment. Adjusted ARRs were 0·17 (95% CI 0·14-0·21) with ozanimod 1·0 mg, 0·22 (0·18-0·26) with ozanimod 0·5 mg, and 0·28 (0·23-0·32) with interferon beta-1a, with rate ratios versus interferon beta-1a of 0·62 (95% CI 0·51-0·77; p<0·0001) for ozanimod 1·0 mg and 0·79 (0·65 to 0·96; p=0·0167) for ozanimod 0·5 mg. The incidence of treatment-emergent adverse events was higher in the interferon beta-1a group (365 [83·0%] of 440 participants) than in the ozanimod 1·0 mg group (324 [74·7%] of 434) or the ozanimod 0·5 mg group (326 [74·3%] of 439). More participants in the interferon beta-1a group had treatment-emergent adverse events leading to treatment discontinuation than in the ozanimod groups. Incidences of infections and serious treatment-emergent adverse events were similar across treatment groups. No cases of ozanimod-related symptomatic reduction in heart rate and no second-degree or third-degree cases of atrioventricular block were reported. INTERPRETATION: In this 24-month phase 3 study in participants with relapsing multiple sclerosis, ozanimod was well tolerated and associated with a significantly lower rate of clinical relapses than intramuscular interferon beta-1a. These findings show the potential of ozanimod as an effective oral therapy for individuals with relapsing multiple sclerosis. FUNDING: Celgene International II.

Safety and efficacy of ozanimod versus interferon beta-1a in relapsing multiple sclerosis (SUNBEAM): a multicentre, randomised, minimum 12-month, phase 3 trial.

BACKGROUND: Ozanimod, a sphingosine 1-phosphate receptor modulator, selectively binds to receptor subtypes 1 and 5 with high affinity. The RADIANCE phase 2 study showed that ozanimod had better efficacy than placebo on MRI measures, with a favourable safety profile, in participants with relapsing multiple sclerosis. The SUNBEAM study aimed to assess the safety and efficacy of ozanimod versus intramuscular interferon beta-1a in participants with relapsing multiple sclerosis. METHODS: SUNBEAM was a randomised, double-blind, double-dummy, active-controlled phase 3 trial done at 152 academic medical centres and clinical practices in 20 countries. We enrolled participants aged 18-55 years with relapsing multiple sclerosis, baseline expanded disability status scale (EDSS) score of 0·0-5·0, and either at least one relapse within the 12 months before screening or at least one relapse within 24 months plus at least one gadolinium-enhancing lesion within 12 months before screening. Participants were randomly assigned 1:1:1 by a blocked algorithm stratified by country and baseline EDSS score to at least 12 months treatment of either once-daily oral ozanimod 1·0 mg or 0·5 mg or weekly intramuscular interferon beta-1a 30 µg. Participants, investigators, and study staff were masked to treatment assignment. The primary endpoint was annualised relapse rate (ARR) during the treatment period and was assessed in the intention-to-treat population. Safety was assessed in all participants according to the highest dose of ozanimod received. This trial is registered at ClinicalTrials.gov, number NCT02294058 and EudraCT, number 2014-002320-27. FINDINGS: Between Dec 18, 2014, and Nov 12, 2015, 1346 participants were enrolled and randomly assigned to ozanimod 1·0 mg (n=447), ozanimod 0·5 mg (n=451), or interferon beta-1a (n=448). 91 (6·8%) participants discontinued the study drug (29 in the ozanimod 1·0 mg group; 26 in the ozanimod 0·5 mg group; and 36 in the interferon beta-1a group). Adjusted ARRs were 0·35 (0·28-0·44) for interferon beta-1a, 0·18 (95% CI 0·14-0·24) for ozanimod 1·0 mg (rate ratio [RR] of 0·52 [0·41-0·66] vs interferon beta-1a; p<0·0001), and 0·24 (0·19-0·31) for ozanimod 0·5 mg (RR 0·69 [0·55-0·86] vs interferon beta-1a; p=0·0013). Few ozanimod-treated participants discontinued treatment because of adverse events (13 [2·9%] who received ozanimod 1·0 mg; seven [1·5%] who received ozanimod 0·5 mg; and 16 [3·6%] who received interferon beta-1a). No first-dose, clinically significant bradycardia or second-degree or third-degree atrioventricular block was reported. The incidence of serious adverse events was low and similar across treatment groups (13 [2·9%] participants who received ozanimod 1·0 mg; 16 [3·5%] who received ozanimod 0·5 mg; and 11 [2·5%] who received interferon beta-1a). No serious opportunistic infections occurred in ozanimod-treated participants. INTERPRETATION: In participants with relapsing multiple sclerosis treated for at least 12 months, ozanimod was well tolerated and demonstrated a significantly lower relapse rate than interferon beta-1a. These findings provide support for ozanimod as an oral therapy for individuals with relapsing multiple sclerosis. FUNDING: Celgene International II.

Multi-database study of multiple sclerosis: identification, validation and description of MS patients in two countries.

OBJECTIVE: To describe the resources and methods used to identify and validate multiple sclerosis (MS) and match non-MS patients in each of the two databases, and to characterize their demographics, comorbidities and concomitant medications. METHODS: This study was conducted in two separate electronic medical databases, the United States Department of Defense (DOD) military health care system and the United Kingdom's Clinical Practice Research Datalink (CPRD) GOLD. We identified patients with a first recorded diagnosis of MS in 2001-2016 (CPRD) or 2004-2017 (DOD) and matched non-MS patients using algorithms appropriate to each database. We describe patient symptoms, comorbidities, and medication use at the time of the MS diagnosis and compared them to the non-MS cohort. RESULTS: We identified 8695 patients with MS and 86,934 matched non-MS patients in the DOD database and 6932 patients with MS and 68,526 matched non-MS patients in CPRD GOLD. Most MS patients were female (around 70%) and were diagnosed before age 60 (88%). MS patients had higher prevalence of depression and other psychiatric conditions at MS diagnosis compared to non-MS patients. Epilepsy, fractures and infections were also more common. MS patients had many expected symptoms and treatments documented in their records prior to the MS diagnosis. CONCLUSION: These results are consistent between the two databases, as well as with previous studies of MS. Future analyses of these patients' experience after MS diagnosis will provide valuable insights into disease and treatment patterns in relation to risk of chronic diseases and mortality.

Pharmacokinetic considerations and efficacy of levofloxacin in an inhalational anthrax (postexposure) rhesus monkey model.

Because the treatment of inhalational anthrax cannot be studied in human clinical trials, it is necessary to conduct efficacy studies using a rhesus monkey model. However, the half-life of levofloxacin was approximately three times shorter in rhesus monkeys than in humans. Computer simulations to match plasma concentration profile, area under the concentration-time curve (AUC), and time above MIC for a human oral dose of 500 mg levofloxacin once a day identified a dosing regimen in rhesus monkeys that would most closely match human exposure: 15 mg/kg followed by 4 mg/kg administered 12 h later. Approximately 24 h following inhalational exposure to approximately 49 times the 50% lethal doses of Bacillus anthracis (Ames strain), monkeys were treated daily with vehicle, levofloxacin, or ciprofloxacin for 30 days. Ciprofloxacin was administered at 16 mg/kg twice a day. Following the 30-day treatment, monkeys were observed for 70 days. Nine of 10 control monkeys died within 9 days of exposure. No clinical signs were observed in fluoroquinolone-treated monkeys during the 30 treatment days. One monkey died 8 days after levofloxacin treatment, and two monkeys from the ciprofloxacin group died 27 and 36 days posttreatment, respectively. These deaths were probably related to the germination of residual spores. B. anthracis was positively cultured from several tissues from the three fluoroquinolone-treated monkeys that died. MICs of levofloxacin and ciprofloxacin from these cultures were comparable to those from the inoculating strain. These data demonstrate that a humanized dosing regimen of levofloxacin was effective in preventing morbidity and mortality from inhalational anthrax in rhesus monkeys and did not select for resistance.

Pharmacokinetic and pharmacodynamic modeling of recombinant human erythropoietin after multiple subcutaneous doses in healthy subjects.

A pharmacokinetic and pharmacodynamic (PK/PD) model for recombinant human erythropoietin (Epoetin alfa) in healthy subjects was developed to describe the time profiles of changes in serum Epoetin alfa and the pharmacological responses of percent reticulocytes, total red blood cell counts, and hemoglobin after single and multiple subcutaneous administration of Epoetin alfa. Data used in the development of the model were obtained from a clinical study carried out in healthy volunteers in which Epoetin alfa was administered either as 150 IU/kg three-times-a-week (t.i.w.) or fixed 40,000 IU weekly (q.w.) doses for 4 weeks. A dual-absorption rate model (fast zero-order and slow first-order inputs) with linear disposition kinetics was used to characterize the pharmacokinetics of erythropoietin after subcutaneous administration. A new catenary cell production and lifespan loss model was used to fit the pharmacodynamic data yielding estimates of SC50, Smax, and other pharmacodynamic parameters. Flip-flop kinetics was apparent in the pharmacokinetics as the absorption rate was slower (k(a) = 0.7 day(-1)) than the elimination rate (CL/V(d) = 1.2-9.2 day(-1)). In the pharmacodynamics, an SC50 of 58 mIU/mL was estimated indicating that low serum erythropoietin concentrations were sufficient to produce pharmacological effects. The established PK/PD model predicts similar pharmacological responses of hemoglobin and total red blood cell counts for the 150 IU/kg t.i.w. and 40,000 IU q.w. regimens in healthy subjects.

Pharmacokinetic and pharmacodynamic modeling of recombinant human erythropoietin after single and multiple doses in healthy volunteers.

This study describes a pharmacokinetic (PK) model to account for serum recombinant human erythropoietin (rHuEpo) concentrations in healthy volunteers following intravenous (IV) and subcutaneous (SC) dosing; it also characterizes the pharmacodynamics (PD) of SC rHuEpo effects on reticulocytes, red blood cells (RBC), and hemoglobin (Hb) in blood. Data were obtained from 4 clinical studies carried out in healthy volunteers. Epoetin alfa (rHuEpo) was administered as 5 single IV doses ranging from 10 to 500 IU/kg, as 8 single SC doses ranging from 300 to 2400 IU/kg, and as 2 multiple SC dosage regimens (150 IU/kg/3 times a week [tiw] and 600 IU/kg/wk). A dual-absorption rate model (fast zero-order and slow first-order inputs) with nonlinear disposition characterized the PK of SC rHuEpo. A high K(m) value was obtained indicating that clearance was mildly nonlinear. Absorption was slow (t(max) approximately 24 hours), and the bioavailability of SC rHuEpo increased with dose (ranging from 46%-100%). A catenary cell production and loss model with a feedback down regulation component was used to fit the reticulocyte data yielding estimates of the stimulatory capacity (S(max)), sensitivity (SC(50)), and life span parameters. These parameters were used for simulations of RBC and Hb profiles. An SC(50) of 27 to 61 IU/L was estimated indicating that low physiological plasma rHuEpo concentrations were sufficient to produce pharmacological effects. No marked sex-dependent differences in clinical responses to rHuEpo therapy were found despite baseline differences. Realistic pharmacokinetic and physiological models accounted for clinical responses from a wide array of dosing conditions with rHuEpo. The rationale for greater efficacy of SC administration of rHuEpo compared to IV was ascertained.