Preclinical Dosimetry, Imaging, and Targeted Radionuclide Therapy Studies of Lu-177-Labeled Albumin-Binding, PSMA-Targeted CTT1403.

PURPOSE: Prostate-specific membrane antigen (PSMA) continues to be the hallmark biomarker for prostate cancer as it is expressed on nearly all prostatic tumors. In addition, increased PSMA expression correlates with castration resistance and progression to the metastatic stage of the disease. Recently, we combined both an albumin-binding motif and an irreversible PSMA inhibitor to develop the novel PSMA-targeted radiotherapeutic agent, CTT1403. This molecule was novel in the field of PSMA-targeted agents as its key motifs resulted in extended blood circulation time and tumor uptake, rapid and extensive internalization into PSMA+ cells, and promising therapeutic efficacy. The objective of this study was to perform IND-enabling translational studies on CTT1403 in rodent models. PROCEDURES: A dose optimization study was performed in PC3-PIP (PSMA+) tumor-bearing mice. Treatment groups were randomly selected to receive one to three 14-MBq injections of CTT1403. Control groups included (1) saline, (2) non-radioactive [175Lu]CTT1403, or (3) two injections of 14 MBq CTT1751, a Lu-177-labeled non-targeted albumin-binding moiety. Tumor growth was monitored up to 120 days. Small-animal single photon emission tomography/X-ray computed tomography imaging was performed with CTT1403 and CTT1751 in PC3-PIP tumor-bearing mice to visualize infiltration of the Lu-177-labeled agent into the tumor. In preparation for a first-in-human study, human absorbed doses were estimated based on rat biodistribution out to 5 weeks to determine a safe CTT1403 therapy dose in humans. RESULTS: Two to 3 injections of 14 MBq CTT1403 yielded significant tumor growth inhibition and increased survival compared with all control groups and mice receiving 1 injection of 14 MBq CTT1403. Five of 12 mice receiving 2 or 3 injections of CTT1403 survived to the 120-day post-treatment study endpoint. Dosimetry identified the kidneys as the dose-limiting organ, with an equivalent dose of 5.18 mSv/MBq, resulting in a planned maximum dose of 4.4 GBq for phase 1 clinical trials. CONCLUSIONS: The preclinical efficacy and dosimetry of CTT1403 suggest that this agent has significant potential to be safe and effective in humans.

Phase I Study of CTT1057, an 18F-Labeled Imaging Agent with Phosphoramidate Core Targeting Prostate-Specific Membrane Antigen in Prostate Cancer.

Agents targeting prostate-specific membrane antigen (PSMA) comprise a rapidly emerging class of radiopharmaceuticals for diagnostic imaging of prostate cancer. Unlike most other PSMA agents with a urea backbone, CTT1057 is based on a phosphoramidate scaffold that irreversibly binds to PSMA. We conducted a first-in-humans phase I study of CTT1057 in patients with localized and metastatic prostate cancer. Methods: Two patient cohorts were recruited. Cohort A patients had biopsy-proven localized prostate cancer preceding radical prostatectomy, and cohort B patients had metastatic castration-resistant prostate cancer. Cohort A patients were imaged at multiple time points after intravenous injection with 362 ± 8 MBq of CTT1057 to evaluate the kinetics of CTT1057 and estimate radiation dose profiles. Mean organ-absorbed doses and effective doses were calculated. CTT1057 uptake in the prostate gland and regional lymph nodes was correlated with pathology, PSMA staining, and the results of conventional imaging. In cohort B, patients were imaged 60-120 min after injection of CTT1057. PET images were assessed for overall image quality, and areas of abnormal uptake were contrasted with conventional imaging. Results: In cohort A (n = 5), the average total effective dose was 0.023 mSv/MBq. The kidneys exhibited the highest absorbed dose, 0.067 mGy/MBq. The absorbed dose of the salivary glands was 0.015 mGy/MBq. For cohort B (n = 15), CTT1057 PET detected 97 metastatic lesions, and 44 of 56 bone metastases detected on CTT1057 PET (78.5%) were also detectable on bone scanning. Eight of 32 lymph nodes positive on CTT1057 PET (25%) were enlarged by size criteria on CT. Conclusion: CTT1057 is a promising novel phosphoramidate PSMA-targeting 18F-labeled PET radiopharmaceutical that demonstrates similar biodistribution to urea-based PSMA-targeted agents, with lower exposure to the kidneys and salivary glands. Metastatic lesions are detected with higher sensitivity on CTT1057 imaging than on conventional imaging. Further prospective studies with CTT1057 are warranted to elucidate its role in cancer imaging.

177Lu-Labeled Phosphoramidate-Based PSMA Inhibitors: The Effect of an Albumin Binder on Biodistribution and Therapeutic Efficacy in Prostate Tumor-Bearing Mice.

Prostate-specific membrane antigen (PSMA) continues to be an active biomarker for small-molecule PSMA-targeted imaging and therapeutic agents for prostate cancer and various non-prostatic tumors that are characterized by PSMA expression on their neovasculature. One of the challenges for small-molecule PSMA inhibitors with respect to delivering therapeutic payloads is their rapid renal clearance. In order to overcome this pharmacokinetic challenge, we outfitted a 177Lu-labeled phosphoramidate-based PSMA inhibitor (CTT1298) with an albumin-binding motif (CTT1403) and compared its in vivo performance with that of an analogous compound lacking the albumin-binding motif (CTT1401). The radiolabeling of CTT1401 and CTT1403 was achieved using click chemistry to connect 177Lu-DOTA-N3 to the dibenzocyclooctyne (DBCO)-bearing CTT1298 inhibitor cores. A direct comparison in vitro and in vivo performance was made for CTT1401 and CTT1403; the specificity and efficacy by means of cellular uptake and internalization, biodistribution, and therapeutic efficacy were determined for both compounds. While both compounds displayed excellent uptake and rapid internalization in PSMA+ PC3-PIP cells, the albumin binding moiety in CTT1403 conferred clear advantages to the PSMA-inhibitor scaffold including increased circulating half-life and prostate tumor uptake that continued to increase up to 168 h post-injection. This increased tumor uptake translated into superior therapeutic efficacy of CTT1403 in PSMA+ PC3-PIP human xenograft tumors.

INO-4995 therapeutic efficacy is enhanced with repeat dosing in cystic fibrosis knockout mice and human epithelia.

Progressive lung damage in cystic fibrosis (CF) has been linked to inadequate airway mucosal hydration. We previously demonstrated that an inositol tetrakisphosphate analog, 1-O-octyl-2-O-butyryl-myo-inositol 3,4,5,6-tetrakisphosphate octakis(propionoxymethyl)ester (INO-4995), regulates airway secretory and absorptive processes, affecting mucosal hydration by prolonged (24 h) inhibition of Na(+) and fluid absorption in CF human nasal epithelia (CFHNE). The objectives of this study were to further assess clinical potential of INO-4995 in CF through ascertaining in vivo activity in mice with CF, determining the effects of repeated administration on potency and determining cytoplasmic half-life. Uptake and metabolism of [(3)H]INO-4995 was monitored with HPLC to calculate intracellular half-life. INO-4995 was administered in vitro repeatedly over 4 to 8 days to CFHNE. Fluid absorption was assessed by blue dextran exclusion, and basal short-circuit current was measured in Ussing chambers. INO-4995 (1-100 microg/kg) was dosed intranasally either as a single dose or once per day over 4 days to gut-corrected CF mice. [(3)H]INO-4995 was rapidly taken up by epithelial cultures and converted to the active drug, which had a half-life of 40 hours. Repeated daily application of INO-4995 to CFHNE lowered the effective concentration for inhibition of fluid absorption and amiloride-sensitive short-circuit current in cultured CFHNE, and reduced nasal potential difference to nearly control levels in gut-corrected CF mice. Ca(2+)-activated Cl(-) channel activity was also boosted in cultures. Mouse nasal levels fell from abnormal levels to within 2 muA of normal with repeated exposure to 0.8 microg/kg over 4 days. These data support further development of INO-4995 for the treatment of CF.