Feasibility Study of Copper-64 Radioisotope Production by Secondary Fast Neutron Bombardment

As a beta and positron emitter, copper-64 (Cu-64) has been coined a theranostic agent in nuclear medicine. Copper-64 is generally produced by bombarding a nickel-64 target with a proton beam via 64Ni(p,n)64Cu nuclear reaction. In this work, secondary fast neutrons are proposed to produce Cu-64 radioisotope via 64Zn(n,p)64Cu nuclear reaction. The secondary fast neutrons were produced by a 10 MeV proton-irradiated primary titanium (Ti) target simulated using the PHITS 3.16 code. In the simulation, the Ti target thickness was varied from 0.01 to 0.1 cm to obtain the optimum secondary fast neutron flux, which was calculated in the rear, radial, and front directions. The Cu-64 radioactivity yield was then computed using the TENDL 2019 nuclear cross-section data. Also, the expected radioactive impurities during Cu-64 production were predicted. The simulation results indicated that the total fast neutron flux resulted from the 10-MeV proton bombarded Be target was 1.70x1012 n/cm2s. The maximum integrated Cu-64 radioactivity yield was 2.33 MBq/µAh when 0.03 cm thick Ti target was shot with 10-MeV protons. The most significant impurities predicted during the bombardment were radioactive isotopes e.g., Co-61, and Zn-65, with the total radioactivity yield estimated to be 0.28 Bq/µAh.

1120. Absorption, Metabolism, and Excretion of [14C]-Tebipenem Pivoxil Hydrobromide (TBP-PI-HBr) Following a Single Oral Dose in Healthy Male Subjects

Background Tebipenem pivoxil hydrobromide (TBP-PI-HBr) is an oral prodrug that is converted to tebipenem (TBP), the active moiety, with activity against multidrug-resistant gram-negative pathogens, including extended-spectrum-β-lactamase (ESBL)-producing Enterobacterales. TBP-PI-HBr is the first oral carbapenem intended for treating complicated urinary tract infections and acute pyelonephritis. This study evaluated the absorption, metabolism, and excretion (AME) of TBP-PI-HBr following a single oral dose of [14C]-TBP-PI-HBr to healthy males and characterized metabolites in plasma, urine, and feces. Methods This was a Phase 1, open-label, single-dose study in healthy subjects. Study drug was provided as radiolabeled and non-radiolabeled active pharmaceutical ingredient containing approximately 150 μCi of [14C]-TBP-PI-HBr. On Day 1, each subject received a 600 mg dose of TBP-PI-HBr. administered with 240 mL of water and fasted overnight for at least 10 hours. Blood samples were collected to determine TBP concentrations (whole blood), total radioactivity (whole blood and plasma), and metabolite profiling and identification were determined from plasma, urine, and feces. For mass balance, total radioactivity derived from urine and feces collections were determined. PK parameters were calculated using noncompartmental methods. Results Total radioactivity in plasma and whole blood decreased rapidly with geometric mean t½ values of 6.0 hours and 3.5 hours, respectively and Tmax of 1 hour. The cumulative mean recovery of radioactivity was 38.7% in urine and 44.6% in feces. Most of the administered radioactivity was recovered in the first 144 hours post dose in urine and feces (80.0%). Six of 8 subjects achieved a mass balance recovery ranging from 80.1% to 85.0%. The TBP plasma to total radioactivity ratio of 0.536 indicated that other metabolites contribute to the total radioactivity AUC in plasma. Metabolite profiling and identification results indicated that TBP was the major component in plasma and urine. The inactive ring open metabolite of TBP (LJC 11,562) was also found in plasma ( >10%), urine (5.27%), and feces ( >10%) as a secondary metabolite. Conclusion This study adequately characterized the AME of TBP-PI-HBr in humans. Disclosures Vipul K. Gupta, Ph.D., Spero Therapeutics (Employee, Shareholder) Gary Maier, PhD, Spero Therapeutics, Inc. (Consultant) Leanne Gasink, MD, Spero Therapeutics, Inc. (Consultant) Amanda Ek, MS, Spero Therapeutics, Inc. (Employee) Mary Fudeman, BA, MBA, Spero Therapeutics, Inc. (Employee) Praveen Srivastava, MS, BS, Spero Therapeutics, Inc. (Employee) Angela Talley, MD, Spero Therapeutics, Inc. (Employee)

Metabolism, Excretion, and Mass Balance of [ 14 C]-Rezafungin in Animals and Humans

Rezafungin is a novel echinocandin being developed for treatment of candidemia and invasive candidiasis and for prevention of invasive fungal disease caused by Candida , Aspergillus , and Pneumocystis spp. in recipients of blood and marrow transplantation. Studies using [ 14 C]-radiolabeled rezafungin were conducted in rats, monkeys, and humans to characterize the mass balance, excretion, and pharmacokinetics of [ 14 C]-rezafungin and to evaluate relative amounts of rezafungin metabolites compared with parent drug. Fecal excretion was the main route of elimination in rats, monkeys, and humans. Radioactivity was primarily excreted as unchanged drug, with ≥95% average total recovery in rats (through 336 hours) and monkeys (through 720 hours). In humans, cumulative recovery of radioactivity through the first 17 days was 52% (38% in feces, 14% in urine) with estimated mean overall recovery through Day 60 of 88.3% (73% in feces, 27% in urine). The clinical pharmacokinetics of rezafungin following a single 400-mg intravenous infusion (200 μCi of [ 14 C]-rezafungin) were similar in plasma, plasma total radioactivity, and whole blood total radioactivity. Unchanged rezafungin represented the majority of total radioactivity in plasma, and the partitioning of total radioactivity into red blood cells was negligible. Across species, rezafungin was primarily metabolized by hydroxylation of the terphenyl, pentyl ether side chain. In these excretion/mass balance, metabolism, and PK studies, clinical observations were consistent with findings in the rat and monkey demonstrating the minimal metabolism and slow elimination of rezafungin after intravenous administration, with fecal excretion as the major route of elimination.

Pharmacokinetics and Disposition of Contezolid in Humans—Resolution of a Disproportionate Human Metabolite for Clinical Development

Contezolid (MRX-I), a novel oxazolidinone antibiotic, was recently approved for the treatment of serious Gram-positive infections. The pharmacokinetics and disposition of [ 14 C]contezolid were investigated in a single-dose human mass balance study. Cross-species comparison of plasma exposure for contezolid and metabolites was performed, and the safety of the disproportionate metabolite in human was evaluated with additional nonclinical studies. After an oral administration of 99.1 μCi/602 mg dose of [ 14 C]contezolid, approximately 91.5% of the radioactivity was recovered in 0–168 h postdose, mainly in urine and followed by feces. The principal metabolic pathway of contezolid in human comprised an oxidative ring opening of 2,3-dihydropyridin-4-one fragment into polar metabolites MRX445-1 and MRX459, with recovery of approximately 48% and 15% of the dose, respectively, in urine and feces. Contezolid, MRX445-1, and MRX459 accounted for 68.0%, 19.5%, and 4.84% of the plasma exposure of the total radioactivity, respectively. Metabolites MRX445-1 and MRX459 were observed in disproportionately higher amounts in human plasma as compared to that rat or dog, the rodent and nonrodent species used for the general nonclinical safety assessment of this molecule. This discrepancy was resolved with additional nonclinical studies, wherein the primary metabolite, MRX445-1, was further characterized. The no observed adverse effect level (NOAEL) of MRX445-1 was determined as 360 mg/kg/day in 14-day repeat-dose test in pregnant and non-pregnant SD rats. Furthermore, MRX445-1 exhibited no antibacterial activity in vitro. Thus, MRX445-1 is not expected to exert clinically relevant pharmacology and toxicity.

The Metabolism and Excretion of the Dipeptidyl Peptidase 4 Inhibitor [14C] Cetagliptin in Healthy Volunteers

Aims: The metabolism and excretion of teneligliptin were investigated in healthy male volunteers after a single oral dose of 100mg/50μCi [14C] cetagliptin. Methods: Plasma, Urine, and feces were collected at regular intervals from six healthy male volunteers, and were analysed for total radioactivity, unchanged cetagliptin and metabolites profile. Results: The highest concentrations in plasma (Cmax) were achieved at 0.75 h postdose. Approximately 53.13% of plasma AUC of total radioactivity was accounted for by parent drug. By 336 h after administration, 91.68% of the administered radioactivity was excreted, and the cumulative excretion in the urine and faeces was 72.88% and 18.81%, respectively. Each metabolite plasma AUC was not higher than 2.93% of plasma AUC of total radioactivity. Four metabolites were detected at trace levels, and it involved hydroxylation (M436-1 and M436-3), N-sulfation (M500), and N-carbamoyl glucuronidation (M640B). These metabolites were detected also in plasma, urine, and feces at low levels, except that metabolite M640B was not detected in feces. No metabolite was observed with >10% of parent compound systemic exposure after oral administration. There were no apparent treatment-related clinically relevant changes in vital signs and clinical laboratory tests. Conclusion: Unchanged cetagliptin was the most abundant radioactive component in all matrices investigated. The primary route of excretion of radioactivity was via the kidneys. There were no major metabolites in plasma. Cetagliptin is a promising DPP-4 inhibitor for the treatment of patients with type 2 diabetes.

Functional model of rat thyroid follicles cultured in Matrigel

Background: Long term maintenance of functional activity of thyroid cells is an essential requirement for basic in vitro studies on the physiology and pathology of the thyroid. An important prerequisite of thyrocytes’ functional activity in vivo and in vitro is their follicle organization. Aim: This study aimed at developing a method of cultivation of functionally active rat thyroid follicles in Matrigel under three-dimensional conditions. Methods: Undamaged rat thyroid follicles were isolated by enzymatic digestion with collagenase/dispase, then embedded into Matrigel, and cultivated for two weeks. Thyroglobulin, thyroxine and zonula occludens-1 (ZO-1) localization were revealed by immunofluorescence analysis. Iodide organification was tested by protein bound 125I (PBI) measurement. Results: Integrity of the follicles was preserved during the whole period of cultivation and was confirmed by 3D reconstruction of ZO-1 localization. Thyroglobulin was detected in thyrocyte cytoplasm, as well as in the intrafollicular lumen. Thyroxine was observed predominantly at the apical side of thyrocytes. Also, generated cultures were characterized by high level of iodide organification: PB125I represented 39 % of the total radioactivity in the Matrigel drop embedding the follicles; at the same time, methimazole almost totally inhibited this process (0.2 % of total radioactivity). Conclusion: The method of rat thyrocyte cultivation in Matrigel, as described here allows to maintain the structural integrity and the functional activity of thyroid follicles in vitro and could be used for wide ranges of basic and applied researches in thyroidology.

Absorption, Distribution, Metabolism, and Excretion of Aprocitentan, a Dual Endothelin Receptor Antagonist, in Humans

Background: Aprocitentan is an orally active, dual endothelin receptor antagonist that may offer a new therapeutic option for the treatment of difficult-to-control hypertension. Objective: To investigate safety, tolerability, mass balance, absorption, distribution, metabolism, and excretion of aprocitentan. Methods: : In this single-center, open-label study a single oral dose of 25 mg containing 3.7 MBq of 14C-radiolabeled aprocitentan was administered to 6 healthy male subjects to investigate safety, tolerability, mass balance, absorption, distribution, metabolism, and excretion of aprocitentan. Metabolites were identified using mass spectrometry and, where possible, confirmed and quantified with reference compounds. Results: Aprocitentan was well tolerated and there were no clinically significant findings for any safety variable. The geometric mean cumulative recovery of radioactivity from urine and feces over 14 days was 77% of the administered radioactive dose, with 52.1% cumulative recovery from urine, and 24.8% from feces. Concentrations of total radioactivity in whole blood were markedly lower compared to plasma. In plasma, 94.3% of total radioactivity was aprocitentan. In urine and feces, 5 and 2, respectively (in feces one being aprocitentan) main products were identified. Metabolism data of aprocitentan identified two main elimination pathways, glucosidation to M3 and hydrolysis to M1, representing approximately 25% and 32% of the radioactive dose, respectively. Conclusions: Based on these metabolism data, aprocitentan can be concomitantly administered without dose adjustment with drugs that are inhibitors or inducers of any metabolizing enzyme, specifically cytochrome P450 enzymes.

Efficacy of systemic administration of riboflavin on a rabbit model of corneal alkali burn

Changes in the barrier mechanisms in the eye should determine the rational route for the administration and dosage of each drug in the treatment of traumatic injuries and other pathologies. The aim of this study was to examine the efficacy of intra-arterial delivery of 14C-riboflavin (as an “indicator”) and compare it with intravenous and intramuscular administration in an animal model of chemical eye burn. 14C-riboflavin (14C-I) was administered by intra-arterial (carotid artery), intravenous (femoral vein) and intramuscular (femoral muscle) routes. The total radioactivity was determined over 2 h in the plasma and structures of the rabbit’s eyes using a scintillation counter. The results of the study show that intravascular administration of 14C-I gives significantly higher concentrations of total radioactivity in the blood and is accompanied by a significant increase in the permeability of the blood-barrier and barrier in eyes suffering from burns. The highest concentration in the plasma and aqueous humour of the anterior chamber of the eye was observed during the first hour with the intra-arterial route of administration of 14C-I in either burnt and unburnt eyes. The distribution of total radioactivity in the structures of the eye over the 2 h of the experiment showed a higher level of the drug under intra-arterial administered in the uveal regions, namely: the iris, ciliary body, choroid, retina and also the sclera and cornea. This experimental model shows that intra-arterial administration can increase the bioavailability of a drug to the structures of the eye within a short period of time.

1286. Pharmacokinetics, Excretion, and Mass Balance of [14C]-Rezafungin Following Intravenous (IV) Administration in Healthy Adults

Background Rezafungin is a once-weekly novel echinocandin antifungal currently in Phase 3 development for treatment of candidemia and invasive candidiasis (ReSTORE) and for prevention of invasive fungal disease caused by Candida, Aspergillus, and Pneumocystis in blood and marrow transplant recipients (ReSPECT). Nonclinical ADME studies in rats and monkeys show rezafungin is primarily excreted unchanged in feces, with urine as a minor route. This study was conducted to characterize the routes of elimination of [14C]-rezafungin and the pharmacokinetics of total radioactivity and plasma rezafungin in humans. Methods Nine healthy male subjects received a single IV 400-mg rezafungin infusion containing 200 µCi of [14C]-rezafungin. Serial blood samples, urine, and feces were collected at specified times over 60 days; subjects were initially confined in the clinical research unit (CRU) for 17 days postdose and returned for two follow-up visits (days 29 and 60). During the period of time subjects were away from the CRU, recovery of radioactivity was estimated by linear interpolation. Results Rezafungin exhibited a long plasma half-life and was mainly excreted in feces unchanged. Cumulative recovery of radioactivity from excreta collected through the first 17 days was 52% (38% in feces, 14% in urine), reinforcing the slow overall elimination of rezafungin. Overall recovery of the administered dose by day 60 was estimated to be 88.3% (65.6% in feces, 22.7% in urine) (Figure 1). Mean blood/plasma concentration ratios ranged from 0.860 to 1.02 through the last collection time point (day 60), which indicated low association of radioactivity with blood cells. Rezafungin was the predominant compound measured in plasma and feces across all collected time points. In the urine, as observed in rat and monkey metabolite profiling studies, low level, inactive, oxidative metabolites were identified as 2-, 3-, 4-hydroxylpentyl rezafungin, and despentyl-rezafungin Figure 1 Conclusion Results from this human excretion balance, metabolism and PK study are consistent with nonclinical results, which showed fecal excretion as the major route of elimination of rezafungin. Rezafungin was the predominant compound in plasma and feces. Disclosures Voon Ong, PhD, Cidara Therapeutics, Inc. (Employee, Shareholder) Shawn Flanagan, PhD, Cidara Therapeutics, Inc. (Employee, Shareholder) Taylor Sandison, MD, MPH, Cidara Therapeutics, Inc. (Employee, Shareholder)

Absorption, Metabolism and Excretion of Surufatinib in Rats and Humans

Background: Surufatinib is a potent small-molecule tyrosine kinase inhibitor and exhibited significant efficacy in the treatment of neuroendocrine tumors in clinical trials. Objective: The absorption, metabolism and excretion of surufatinib were investigated in rats and human volunteers following a single oral dose of [14C] surufatinib. Methods: The radioactivity was measured in plasma, urine, feces and bile by liquid scintillation counting, and the metabolites were characterized by liquid chromatography coupled to mass spectrometry. Results: Surufatinib was orally absorbed similarly in rats and human volunteers, with the median Tmax of 4 hours post-dose. The estimated t1/2 appeared longer in humans than in rats (mean t1/2: 3.12 hour for male rats, 6.48 hours for female rats and 23.3 hours for male human volunteers). The excretion of surufatinib was almost complete in rats and human volunteers in the studies, with the total radioactivity recovery of >90% of the dose. Similarly, in rats and humans, fecal excretion predominated (approximately 87% of the dose recovered in feces and only 5% in urine). The parent drug was the major radioactive component detected in the plasma extracts of rats and humans, and no single circulating metabolite accounted for >10% of the total radioactivity. Unchanged drug was a minor radioactive component in the excreta of rats and humans. Conclusion: Fecal excretion was the predominant way for the elimination of surufatinib and its metabolites in rats and humans. No disproportionate circulating metabolite was observed in humans.