R. Keith Frank

Design and synthesis of 225Ac radioimmunopharmaceuticals.

The alpha-particle-emitting radionuclides 213Bi, 211At, 224Ra are under investigation for the treatment of leukemias, gliomas, and ankylosing spondylitis, respectively. 213Bi and 211At were attached to monoclonal antibodies and used as targeted immunotherapeutic agents while unconjugated 224Ra chloride selectively seeks bone. 225Ac possesses favorable physical properties for radioimmunotherapy (10d half-life and 4 net alpha particles), but has a history of unfavorable radiolabeling chemistry and poor metal-chelate stability. We selected functionalized derivatives of DOTA as the most promising to pursue from out of a group of potential 225Ac chelate compounds. A two-step synthetic process employing either MeO-DOTA-NCS or 2B-DOTA-NCS as the chelating moiety was developed to attach 225Ac to monoclonal antibodies. This method was tested using several different IgG systems. The chelation reaction yield in the first step was 93+/-8% radiochemically pure (n=26). The second step yielded 225Ac-DOTA-IgG constructs that were 95+/-5% radiochemically pure (n=27) and the mean percent immunoreactivity ranged from 25% to 81%, depending on the antibody used. This process has yielded several potential novel targeted 225Ac-labeled immunotherapeutic agents that may now be evaluated in appropriate model systems and ultimately in humans.

Comparison of pretargeted and conventional CC49 radioimmunotherapy using 149Pm, 166Ho, and 177Lu.

The therapeutic efficacies of radiolabeled biotin, pretargeted by monoclonal antibody (mAb)-streptavidin fusion protein CC49 scFvSA, were compared to those of radiolabeled mAb CC49, using the three radiolanthanides in an animal model of human colon cancer. The purpose of the present study was to compare antibody pretargeting to conventional radioimmunotherapy using (149)Pm, (166)Ho, or (177)Lu. Nude mice bearing LS174T colon tumors were injected sequentially with CC49 scFvSA, the blood clearing agent biotin-GalNAc(16), and (149)Pm-, (166)Ho-, or (177)Lu-DOTA-biotin. Tumor-bearing mice were alternatively administered (149)Pm-, (166)Ho-, or (177)Lu-MeO-DOTA-CC49. Therapy with pretargeted (149)Pm-,(166)Ho-, and (177)Lu-DOTA-biotin increased the median time of progression to a 1 g tumor to 50, 41, and 50 days post-treatment, respectively. Therapy with (149)Pm-,(166)Ho-, and (177)Lu-MeO-DOTA-CC49 increased the median time to progression to 53, 24, and 67 days post-treatment, respectively. In contrast, saline controls showed a median time to progression of 13 days postinjection. Treatment with pretargeted (149)Pm-, (166)Ho-, and (177)Lu-biotin or (149)Pm-, (166)Ho-, and (177)Lu-CC49 increased tumor doubling time to 18-36 days, compared to 3 days for saline controls. Among treated mice, 23% survived >84 days post-therapy, and 11% survived 6 months, but controls survived <29 days. Long-term survivors showed tumor growth inhibition or partial regression, extensive necrosis in residual masses, and no evidence of nontarget tissue toxicity at necropsy. Both pretargeted and conventional RIT demonstrated considerable efficacy in an extremely aggressive animal model of cancer. Our results identified (177)Lu as an optimal radiolanthanide for future evaluation of these agents in toxicity and multiple-dose therapy studies.

Methods of producing high specific activity Sn-117m with commercial cyclotrons

Sn-117m is a theragnostic isotope of increasing interest. It has ideal imaging and therapeutic properties for many applications. Production of high specific activity Sn-117m can only be achieved with accelerators and the Sb(p,x) and Cd-116(α,3n) reactions provide the highest yields for this isotope. This paper describes the production and a new purification method for obtaining commercial quantities of Sn-117m via the Cd-116(α,3n) reaction. The purified Sn-117m has subsequently been chelated with an aminobenzyl derivative of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and then conjugated with annexin V-128 to produce an agent for the diagnosis and treatment of vulnerable plaque.

A preclinical investigation of the saturation and dosimetry of 153Sm-DOTMP as a bone-seeking radiopharmaceutical.

INTRODUCTION The therapeutic potential of the bone-seeking radiopharmaceutical 153Sm-labeled 1,4,7,10-tetraazacyclododecanetetramethylenephosphonic acid (153Sm-DOTMP) was assessed by measuring its dosage-dependent skeletal uptake at two chelant-to-metal ratios and its source organ residence times at a chelant-to-metal ratio of 1.5:1. A similar agent, 153Sm-labeled ethylenediaminetetramethylenephosphonic acid (153Sm-EDTMP), has been reported to exhibit dosage-limiting skeletal saturation. METHODS Sm-DOTMP was prepared with tracer activity of 153Sm and sufficient stable, unenriched Sm to simulate different activities. Cohorts of seven 280-g Sprague-Dawley rats were administered the equivalent of 296, 592, 888, 1184 and 1480 MBq (8, 16, 24, 32 and 40 mCi) at a fixed chelant-to-metal ratio of 1.5:1 and euthanized 3 h after administration. Cohorts of three 128-g Sprague-Dawley rats were administered equivalent dosages of 10.4, 592 and 888 (0.28, 16 and 32 mCi) at a fixed chelant-to-metal ratio of 270:1 and euthanized 2 h after administration. A simulated activity of 1480 MBq (40 mCi) at a chelant-to-metal ratio of 1.5:1 was administered to cohorts of seven rats that were euthanized at 2, 4, 24 or 48 h postadministration. The heart, lungs, liver, spleen, kidneys, small intestine, large intestine, urinary bladder, muscle and a femur were excised, weighed and counted. The data were analyzed to determine skeletal uptake and source organ residence times. RESULTS No statistically significant skeletal saturation was observed up to human-equivalent dosages of 370 GBq (10 Ci) at a chelant-to-metal ratio of 1.5:1, but the skeletal uptake dropped by 40% over the range of dosages at a chelant-to-metal ratio of 270:1. At a chelant-to-metal ratio of 1.5:1, the preferred ratio, the skeletal uptake fraction in rats was 0.408 (95% confidence interval 0.396-0.419) with an effective half-life of 47.3 h (95% confidence interval 42.3-53.7; the physical half-life of 153Sm is 46.3 h). Extrapolating to an adult human model, 52.9 GBq (1.43 Ci) of 153Sm-DOTMP would deliver 40 Gy to the red marrow. CONCLUSION 153Sm-DOTMP has dosimetry equivalent to that of 153Sm-EDTMP at low dosages, yet with no skeletal saturation at higher administered activities.

Dosimetry of a 90Y-hydroxide liquid brachytherapy treatment approach to canine osteosarcoma using PET/CT

A new treatment strategy based on direct injections of (90)Y-hydroxide into the tumor bed in dogs with osteosarcoma was studied. Direct injections of the radiopharmaceutical into the tumor bed were made according to a pretreatment plan established using (18)F-FDG images. Using a special drill, cannulas were inserted going through tissue, tumor and bone. Using these cannulas, direct injections of the radiopharmaceutical were made. The in vivo biodistribution of (90)Y-hydroxide and the anatomical tumor bed were imaged using a time-of-flight (TOF) PET/CT scanner. The material properties of the tissues were estimated from corresponding CT numbers using an electron-density calibration. Radiation absorbed dose estimates were calculated using Monte Carlo methods where the biodistribution of the pharmaceutical from PET images was sampled using a collapsing 3-D rejection technique. Dose distributions in the tumor bed and surrounding tissues were calculated, showing significant heterogeneity with multiple hot spots at injection sites. Dose volume histograms showed that approximately 33.9% of bone and tumor and 70.2% of bone marrow and trabecular bone received an absorbed dose over 200Gy; approximately 3.2% of bone and tumor and 31.0% of bone marrow and trabecular bone received a total dose of over 1000Gy.