Characterizing the Pharmacokinetics and Biodistribution of Therapeutic Proteins: An Industry White Paper.

Characterization of the pharmacokinetics and biodistribution of therapeutic proteins (TPs) is a hot topic within the pharmaceutical industry, particularly with an ever-increasing catalog of novel modality TPs. Here, we review the current practices, and provide a summary of extensive cross-company discussions as well as a survey completed by International Consortium for Innovation and Quality members on this theme. A wide variety of in vitro, in vivo and in silico techniques are currently used to assess pharmacokinetics and biodistribution of TPs, and we discuss the relevance of these from an industry perspective, focusing on pharmacokinetic/pharmacodynamic understanding at the preclinical stage of development, and translation to human. We consider that the 'traditional in vivo biodistribution study' is becoming insufficient as a standalone tool, and thorough characterization of the interaction of the TP with its target(s), target biology, and off-target interactions at a microscopic scale are key to understand the overall biodistribution on a full-body scale. Our summary of the current challenges and our recommendations to address these issues could provide insight into the implementation of best practices in this area of drug development, and continued cross-company collaboration will be of tremendous value. SIGNIFICANCE STATEMENT: The Innovation and Quality Consortium Translational and ADME Sciences Leadership Group working group for the absorption, distribution, metabolism, and excretion of therapeutic proteins evaluates the current practices and challenges in characterizing the pharmacokinetics and biodistribution of therapeutic proteins during drug development, and proposes recommendations to address these issues. Incorporating the in vitro, in vivo and in silico approaches discussed herein may provide a pragmatic framework to increase early understanding of pharmacokinetic/pharmacodynamic relationships, and aid translational modeling for first-in-human dose predictions.

Absorption, Distribution, Metabolism, and Excretion of Therapeutic Proteins: Current Industry Practices and Future Perspectives.

Therapeutic proteins (TPs) comprise a variety of modalities, including antibody-based drugs, coagulation factors, recombinant cytokines, enzymes, growth factors, and hormones. TPs usually cannot traverse cellular barriers and exert their pharmacological activity by interacting with targets on the exterior membrane of cells or with soluble ligands in the tissue interstitial fluid/blood. Due to their large size, lack of cellular permeability, variation in metabolic fate, and distinct physicochemical characteristics, TPs are subject to different absorption, distribution, metabolism, and excretion (ADME) processes as compared with small molecules. Limited regulatory guidance makes it challenging to determine the most relevant ADME data required for regulatory submissions. The TP ADME working group was sponsored by the Translational and ADME Sciences Leadership Group within the Innovation and Quality (IQ) consortium with objectives to: (1) better understand the current practices of ADME data generated for TPs across IQ member companies, (2) learn about their regulatory strategies and interaction experiences, and (3) provide recommendations on best practices for conducting ADME studies for TPs. To understand current ADME practices and regulatory strategies, an industry-wide survey was conducted within IQ member companies. In addition, ADME data submitted to the U.S. Food and Drug Administration was also collated by reviewing regulatory submission packages of TPs approved between 2011 and 2020. This article summarizes the key learnings from the survey and an overview of ADME data presented in biologics license applications along with future perspectives and recommendations for conducting ADME studies for internal decision-making as well as regulatory submissions for TPs. SIGNIFICANCE STATEMENT: This article provides comprehensive assessment of the current practices of absorption, distribution, metabolism, and excretion (ADME) data generated for therapeutic proteins (TPs) across the Innovation and Quality participating companies and the utility of the data in discovery, development, and regulatory submissions. The TP ADME working group also recommends the best practices for condu-cting ADME studies for internal decision-making and regulatory submissions.

Secukinumab dosing every 2 weeks demonstrated superior efficacy compared with dosing every 4 weeks in patients with psoriasis weighing 90 kg or more: results of a randomized controlled trial.

BACKGROUND: Obesity is a common comorbidity of psoriasis and can attenuate response to biologic treatment. OBJECTIVES: To investigate the efficacy, safety and tolerability of secukinumab 300 mg every 2 weeks (Q2W) vs. secukinumab 300 mg every 4 weeks (Q4W) in patients with a higher bodyweight. METHODS: In this multicentre, double-blind, parallel-group trial, 331 patients with moderate-to-severe chronic plaque psoriasis weighing ≥ 90 kg were randomized to receive secukinumab 300 mg Q2W or secukinumab 300 mg Q4W. Patients who did not achieve Psoriasis Area and Severity Index (PASI) 90 at week 16 on the Q4W regimen were reallocated to remain on the Q4W regimen or uptitrate to Q2W. RESULTS: At week 16, Q2W dosing (n = 165) led to significantly higher PASI 90 responses vs. Q4W [n = 166; 73.2% vs. 55.5%, one-sided P-value = 0.0003, odds ratio estimate (95% confidence intervals): 2.3 (1.4-3.8)]. At week 52, higher efficacy responses were maintained in the Q2W arm (n = 165) vs. Q4W (n = 83); PASI 75: 88.9% vs. 74.8%; PASI 90: 76.4% vs. 52.4%; PASI 100: 46.7% vs. 27.3%; Investigator's Global Assessment 0/1: 75.9% vs. 55.6% and Dermatology Life Quality Index 0/1: 66.1% vs. 48.8%. PASI 90 nonresponders at week 16 who uptitrated to Q2W (n = 31) showed higher efficacy responses at week 32 (16 weeks post-uptitration, PASI 90: 38.7% vs. 16.5%) vs. those who remained on Q4W (n = 40). Safety results were comparable across treatment arms and consistent with the established secukinumab safety profile. CONCLUSIONS: Secukinumab 300 mg Q2W demonstrated superior and sustained efficacy compared with Q4W in patients with moderate-to-severe plaque psoriasis weighing ≥ 90 kg. PASI 90 nonresponders derived additional benefits from uptitration to a Q2W regimen (ClinicalTrials.gov identifier: NCT03504852). What is already known about this topic? Obesity is a common comorbidity of psoriasis and can attenuate response to biologic treatment. Secukinumab is a fully human monoclonal antibody that selectively neutralizes interleukin-17A in the treatment of moderate-to-severe plaque psoriasis. Subgroup analyses of previous study results and pharmacokinetic/pharmacodynamic modelling data suggest that heavier patients may benefit from higher exposure to secukinumab through an increased dosing frequency [300 mg every 2 weeks (Q2W) vs. every 4 weeks (Q4W)]. What does this study add? Over 52 weeks, secukinumab 300 mg Q2W demonstrated superior efficacy compared with secukinumab 300 mg Q4W in patients with moderate-to-severe plaque psoriasis weighing ≥90 kg, with comparable safety results, consistent with the established secukinumab safety profile. In patients who did not achieve PASI 90 at week 16 on the Q4W regimen, uptitration to the Q2W regimen at week 16 resulted in improved efficacy responses through week 52 after switching.

Secukinumab demonstrates efficacy, safety, and tolerability upon administration by 2 ml autoinjector in adult patients with plaque psoriasis: 52-week results from MATURE, a randomized, placebo-controlled trial.

Convenient administration is an important factor for treatment adherence in patients with psoriasis. MATURE study reports the efficacy, safety, tolerability, and pharmacokinetics (PKs) of secukinumab 300 mg 2 ml autoinjector (AI) from MATURE trial (NCT03589885). Eligible patients were randomized to secukinumab 300 mg 2 ml AI or 2× 1 ml prefilled syringe (PFS) or placebo. The co-primary endpoints were psoriasis area and severity index (PASI) 75 and investigator's global assessment (IGA) 0/1 response rates at Week 12 versus placebo. Other endpoints included PASI90/100 response, dermatology life quality index (DLQI) 0/1, PKs, 2 ml AI usability rated using self-injection assessment questionnaire (SIAQ), and safety. The study met both co-primary and secondary endpoints (p < 0.0001). Secukinumab 300 mg 2 ml AI and 2× 1 ml PFS treatments led to superior PASI75/90/100 (2 ml AI: 95.1%/75.6%/43.9%; 2× 1 mL PFS: 83.2%/62.6%/37.5% and placebo: 10%/5.0%/0.0%, respectively), IGA, and DLQI 0/1 responses compared with placebo, and efficacy was sustained through 52 weeks. SIAQ results showed high usability of self-injection with 2 mL AI device. No new safety signals were observed. Study design may bias the interpretation of safety profile after Week 12, due to different exposure of secukinumab versus placebo. Secukinumab 300 mg administered with the 2 mL AI demonstrated superior efficacy over placebo, good tolerability, and convenient administration.

Comparison of pharmacokinetics, safety and tolerability of secukinumab administered subcutaneously using different delivery systems in healthy volunteers and in psoriasis patients.

AIMS: The aim of the study was to compare the pharmacokinetics (PK), safety and tolerability of secukinumab with different devices for subcutaneous (s.c.) administration of 2 mL. METHODS: A phase 1 study in healthy subjects with 6 devices to administer 2 mL injection volumes was conducted to evaluate the serum PK, safety and tolerability of secukinumab following single s.c. injection of 300 mg in the abdomen (either side) or in the thigh (either leg). Primary PK endpoints were maximum observed serum concentration and area under the serum concentration-time curve. The impact of device, site and side of injection on serum exposure was evaluated. In a phase 3 study in psoriasis patients, PK of secukinumab was evaluated following multiple s.c. injections of 300 mg by either 2 × 1-mL prefilled syringe or 1 × 2-mL prefilled syringe. RESULTS: Mean serum concentration-time profiles for administration as 2 × 1 mL injections or as 1 × 2 mL injections were similar. With an injection volume of 2 mL, perceived injection pain was not different from 2 × 1 mL injections. A nonclinically significant difference in PK endpoints was observed between thigh and abdomen. Results with a 2 mL prefilled syringe in a 1-year phase 3 study in patients confirmed PK results observed in the phase 1 study. CONCLUSION: Collective evidence from both studies demonstrated that 2-mL injections of secukinumab into the abdomen or thigh using different devices resulted in comparable PK characteristics and were all well tolerated without noticeable local reactions.

ß-Defensin 2 is a responsive biomarker of IL-17A-driven skin pathology in patients with psoriasis.

BACKGROUND: IL-17A is a key driver of human autoimmune diseases, particularly psoriasis. OBJECTIVE: We sought to determine the role of IL-17A in psoriasis pathogenesis and to identify a robust and measurable biomarker of IL-17A-driven pathology. METHODS: We studied 8 healthy subjects and 8 patients with psoriasis before and after administration of secukinumab, a fully human anti-IL-17A mAb, and used a combination of classical techniques and a novel skin microperfusion assay to evaluate the expression of 170 proteins in blood, nonlesional skin, and lesional skin. For validation, we also tested stored sera from 601 patients with a variety of autoimmune diseases. RESULTS: IL-17A was specifically expressed in lesional compared with nonlesional psoriatic skin (9.8 vs 0.8 pg/mL, P < .001). Proteomic and gene transcription analyses revealed dysregulated antimicrobial peptides, proinflammatory cytokines, and neutrophil chemoattractants, levels of which returned to normal after treatment with secukinumab. ß-Defensin 2 (BD-2) was identified as a biomarker of IL-17A-driven pathology by comparing protein expression in patients with psoriasis versus that in healthy subjects (5746 vs 82 pg/mL in serum, P < .0001; 2747 vs <218 pg/mL in dermis, P < .001), responsiveness to secukinumab therapy, and synergistic induction by IL-17A and TNF-α in epidermal keratinocytes. In a validation set of sera from 601 patients with autoimmune diseases thought to be IL-17A driven, we found that BD-2 levels are most highly increased in patients with psoriatic skin lesions, and in patients with psoriasis, BD-2 levels correlated well with IL-17A levels (r = 0.70, n = 199, P < .001) and Psoriasis Area and Severity Index scores (r = 0.53, n = 281, P < .001). CONCLUSION: IL-17A is a primary driver of skin pathology in patients with psoriasis, and serum BD-2 is an easily measurable biomarker of IL-17A-driven skin pathology.

Visualisation and quantification of subcutaneous injections of different volumes, viscosities and injection rates: An ex-vivo micro-CT study.

The effects of subcutaneous (SC) injection parameters such as drug formulation volume, viscosity and injection rate on therapeutic performance and tolerability have not been established for any drug product. In this study four groups of SC injections were performed on fresh ex vivo minipig abdominal tissue samples, varying volume (0.5-1 mL), viscosity (1-11 cP) and rate (0.02-0.1 mL/s). Micro-CT provided high resolution (50 micron) imaging of the SC tissues before and after injection, enabling a detailed 3D visualisation and analysis of how both injection parameters and tissue microstructure influence spatial distribution of injectables. We found that volume was the only significant factor for spatial distribution of injectate within our design space, and there were no significant factors for tissue backpressure. Variability within test groups was typically greater than differences between group means. Accordingly, whilst the higher viscosity formulations consistently exhibited reduced spatial distribution, the sample size was not large enough to establish confidence in this result. Comparing our findings to clinical evidence, we conclude that injection site and depth are more likely to influence PK and bioavailability than volume, viscosity and rate within our experimental space.

Secukinumab, a human anti-IL-17A monoclonal antibody, for moderate to severe Crohn's disease: unexpected results of a randomised, double-blind placebo-controlled trial.

OBJECTIVE: The authors tested whether the anti-interleukin (IL)-17A monoclonal antibody secukinumab was safe and effective for the treatment of active Crohn's disease. DESIGN: In a double-blind, randomised, placebo-controlled proof-of-concept study, 59 patients with moderate to severe Crohn's disease (Crohn's Disease Activity Index (CDAI) ≥220 to ≤450) were assigned in a 2:1 ratio to 2×10 mg/kg intravenous secukinumab or placebo. The primary end point, addressed by bayesian statistics augmented with historical placebo information, was the probability that secukinumab reduces the CDAI by ≥50 points more than placebo at week 6. Ancillary analyses explored associations of 35 candidate genetic polymorphisms and faecal calprotectin response. RESULTS: 59 patients (39 secukinumab, 20 placebo, mean baseline CDAI 307 and 301, respectively) were recruited. 18/59 (31%) patients discontinued prematurely (12/39 (31%) secukinumab, 6/20 (30%) placebo), 10/59 (17%) due to insufficient therapeutic effect (8/39 (21%) secukinumab, 2/20 (10%) placebo). Fourteen serious adverse events occurred in 10 patients (seven secukinumab, three placebo); 20 infections, including four local fungal infections, were seen on secukinumab versus none on placebo. Primary end point analysis estimated <0.1% probability (CDAI (SD) =33.9 (19.7), 95% credible interval -4.9 to 72.9) that secukinumab reduces CDAI by ≥50 points more than placebo. Secondary area under the curve analysis (weeks 4-10) showed a significant difference (mean ΔCDAI=49; 95% CI (2 to 96), p=0.043) in favour of placebo. Post hoc subgroup analysis showed that unfavourable responses on secukinumab were driven by patients with elevated inflammatory markers (CRP≥10 mg/l and/or faecal calprotectin≥200 ng/ml; mean ΔCDAI=62; 95% CI (-1 to 125), p=0.054 in favour of placebo). Absence of the minor allele of tumour necrosis factor-like ligand 1A was strongly associated with lack of response measured by baseline-adjusted changes in calprotectin at week 6 (p=0.00035 Bonferroni-corrected). CONCLUSIONS: Blockade of IL-17A was ineffective and higher rates of adverse events were noted compared with placebo. CLINICAL TRIAL REGISTRATION: This trial was registered at ClinicalTrial.gov with the number NCT01009281.

Efficacy, tolerability, patient usability, and satisfaction with a 2 mL pre-filled syringe containing secukinumab 300 mg in patients with moderate to severe plaque psoriasis: results from the phase 3 randomized, double-blind, placebo-controlled ALLURE study.

BACKGROUND: Evidence shows good tolerability in patients for subcutaneous injection volumes up to 3 mL. OBJECTIVES: We investigated efficacy, pharmacokinetics, and tolerability of secukinumab 300 mg/2 mL pre-filled syringe (PFS) in patients with moderate to severe plaque psoriasis. METHODS: ALLURE was a 52-week, multicenter, randomized (1:1:1), double-blind, placebo-controlled, parallel-group study. Co-primary endpoints were secukinumab Psoriasis Area Severity Index (PASI) 75 and Investigator's Global Assessment modified 2011 0/1 (IGA mod 2011 0 or 1) responses at week 12 versus placebo. Other endpoints included the Self-Injection Assessment Questionnaire (SIAQ), and the ability to follow the instructions for use (IFU). RESULTS: Overall, 214 patients were randomized. The secukinumab 300 mg/2 mL PFS showed superiority over placebo for both PASI 75 (88.9% versus 1.7%; p<.0001) and IGA mod 2011 0 or 1 (76.4% versus 1.4%; p<.0001) responses at week 12. All secondary efficacy endpoints were met. The SIAQ scores were similar across groups and improved similarly over 12 weeks. All patients completed critical steps in the IFU at week 1. CONCLUSIONS: The secukinumab 300 mg/2 mL PFS groups showed superiority versus placebo, and it was a safe, effective, and convenient option for patients with psoriasis. NCT02748863.

Secukinumab for the treatment of psoriasis, psoriatic arthritis, and axial spondyloarthritis: Physical and pharmacological properties underlie the observed clinical efficacy and safety.

Psoriasis, psoriatic arthritis, and axial spondyloarthritis are systemic inflammatory diseases, each commonly manifesting as a spectrum of symptoms, complications, and comorbidities that arise differently in individual patients. Drugs targeting inflammatory cytokines common to the pathogenesis of each of these conditions have been developed, although their specific actions in the different tissues involved are variable. For a drug to be effective, it must be efficiently delivered to and locally bioactive in disease-relevant tissues. Detailed clinical data shed light on the therapeutic effects of individual biologics on specific domains or clinical manifestations of disease and assist in guiding treatment decisions. Pharmacologic, molecular, and functional properties of drugs strongly impact their observed safety and efficacy, and an understanding of these properties provides complementary insight. Secukinumab, a fully human monoclonal IgG1/κ antibody selectively targeting interleukin (IL)-17A, has been in clinical use for >6 years in the treatment of moderate to severe psoriasis, psoriatic arthritis, and both radiographic (also known as ankylosing spondylitis) and nonradiographic axial spondyloarthritis. In this review, we discuss pharmacokinetic and pharmacodynamic data for secukinumab to introduce clinicians to the pharmacological properties of this widely used drug. Understanding how these properties affect the observed clinical efficacy, safety, and tolerability of this drug in the treatment of IL-17A-mediated systemic inflammatory diseases is important for all physicians treating these conditions.

Pharmacokinetics, metabolism, and disposition of deferasirox in beta-thalassemic patients with transfusion-dependent iron overload who are at pharmacokinetic steady state.

Deferasirox (ICL670) is a novel once-daily, orally administered iron chelator to treat chronic iron overload in patients with transfusion-dependent anemias. Absorption, distribution, metabolism, and excretion of [14C]deferasirox at pharmacokinetic steady state was investigated in five adult beta-thalassemic patients. Deferasirox (1000 mg) was given orally once daily for 6 days to achieve steady state. On day 7, patients received a single oral 1000-mg dose (approximately 20 mg/kg) of [14C]deferasirox (2.5 MBq). Blood, plasma, feces, and urine samples collected over 7 days were analyzed for radioactivity, deferasirox, its iron complex Fe-[deferasirox]2, and metabolites. Deferasirox was well absorbed. Deferasirox and its iron complex accounted for 87 and 10%, respectively, of the radioactivity in plasma (area under the curve at steady state). Excretion occurred largely in the feces (84% of dose), and 60% of the radioactivity in the feces was identified as deferasirox. Apparently unchanged deferasirox in feces was partly attributable to incomplete intestinal absorption and partly to hepatobiliary elimination of deferasirox (including first-pass elimination) and of its glucuronide. Renal excretion was only 8% of the dose and included mainly the glucuronide M6. Oxidative metabolism by cytochrome 450 enzymes to M1 [5-hydroxy (OH) deferasirox, presumably by CYP1A] and M4 (5'-OH deferasirox, by CYP2D6) was minor (6 and 2% of the dose, respectively). Direct and indirect evidence indicates that the main pathway of deferasirox metabolism is via glucuronidation to metabolites M3 (acyl glucuronide) and M6 (2-O-glucuronide).

Secukinumab Immunogenicity over 52 Weeks in Patients with Psoriatic Arthritis and Ankylosing Spondylitis.

OBJECTIVE: Secukinumab, a fully human antiinterleukin 17A monoclonal antibody, is efficacious for the treatment of psoriatic arthritis (PsA) and ankylosing spondylitis (AS). This study examined the immunogenicity of secukinumab in patients with PsA and AS exposed to secukinumab for up to 52 weeks. METHODS: Antibody bridging assays were used to assess the immunogenicity of secukinumab in patients with PsA [FUTURE 1-3 studies, and AS (MEASURE 1-4 studies)]. Evaluations were at baseline and at weeks 16 (AS only), 24, and 52. Treatment-emergent antidrug antibodies (TE-ADA) were defined as a positive ADA signal in ≥ 1 posttreatment sample in patients negative at baseline. Positive samples were analyzed for drug-neutralizing potential, and effect of TE-ADA on secukinumab pharmacokinetics, immunogenicity-related adverse events (AE), and efficacy through Week 52 were assessed. RESULTS: Of 1414 treated PsA and 1164 treated AS patients with samples available for immunogenicity evaluation, 5 (0.35%) and 8 (0.69%), respectively, developed TE-ADA. All but 1 PsA patient were biologic-naive; two of the 5 PsA and one of the 8 AS patients received concomitant methotrexate, and two of the 8 AS patients received concomitant sulfasalazine. Associations between TE-ADA and secukinumab dose, frequency, or administration mode were not observed. Other than one PsA patient, all TE-ADA were non-neutralizing. No TE-ADA were associated with any AE. All TE-ADA were associated with normal secukinumab pharmacokinetics and none were associated with loss of secukinumab efficacy. CONCLUSION: Secukinumab treatment was associated with a low (< 1%) incidence of immunogenicity in patients with PsA or AS. (clinicaltrials.gov: NCT01392326; NCT01752634; NCT01989468; NCT01358175; NCT01649375; NCT02008916; NCT02159053).

Secukinumab Treatment Does Not Alter the Pharmacokinetics of the Cytochrome P450 3A4 Substrate Midazolam in Patients With Moderate to Severe Psoriasis.

This open-label disease-drug-drug interaction study assessed whether blockade of the interleukin (IL)-17A pathway by secukinumab and subsequent downregulation of inflammatory cytokines like IL-6 or high-sensitivity C-reactive protein affects the pharmacokinetics (PKs) of a sensitive probe substrate of the cytochrome P450 3A4 isoform (CYP3A4). The PKs of midazolam, metabolized by CYP3A4, was evaluated before and after 7 and 35 days of treatment initiation of subcutaneous secukinumab at a dose of 300 mg weekly in 24 patients with moderate-to-severe psoriasis. Although demonstrating the expected decrease in downstream inflammatory cytokines, secukinumab had no clinically relevant effects on the PKs of midazolam, provided substantial clinical benefit, and was generally well tolerated. In summary, blockade of IL-17A signaling in patients with moderate-to-severe psoriasis does not significantly affect CYP3A4 enzyme activities and, therefore, the use of secukinumab is unlikely to influence the PKs of CYP3A4 substrates.

Pharmacokinetics, distribution, metabolism, and excretion of deferasirox and its iron complex in rats.

Deferasirox (Exjade, ICL670, CGP72670) is an iron-chelating drug for p.o. treatment of transfusional iron overload in patients with beta-thalassemia or sickle cell disease. The pharmacokinetics and disposition of deferasirox were investigated in rats. The animals received single intravenous (10 mg/kg) or p.o. (10 or 100 mg/kg) doses of 14C-radiolabeled deferasirox. Biological samples were analyzed for radioactivity (liquid scintillation counting, quantitative whole-body autoradioluminography), for deferasirox and its iron complex [high-performance liquid chromatography (HPLC)/UV], and for metabolites (HPLC with radiodetection, liquid chromatography/mass spectrometry, 1H and 13C NMR, and two-dimensional NMR techniques). At least 75% of p.o.-dosed deferasirox was absorbed. The p.o. bioavailability was 26% at the 10 mg/kg dose and showed an overproportional increase at the 100 mg/kg dose, probably because of saturation of elimination processes. Deferasirox-related radioactivity was distributed mainly to blood, excretory organs, and gastrointestinal tract. Enterohepatic recirculation of deferasirox was observed. No retention occurred in any tissue. The placental barrier was passed to a low extent. Approximately 3% of the dose was transferred into the breast milk. Excretion of deferasirox and metabolites was rapid and complete within 7 days. Key clearance processes were hepatic metabolism and biliary elimination via multidrug resistance protein 2. Deferasirox, iron complex, and metabolites were excreted largely via bile and feces (total > or = 90%). Metabolism included glucuronidation at the carboxylate group (acyl glucuronide M3) and at phenolic hydroxy groups, as well as, to a lower degree, cytochrome P450-catalyzed hydroxylations. Two hydroxylated metabolites (M1 and M2) were administered to rats and were shown not to contribute substantially to iron elimination in vivo.