Pretargeted radioimmunotherapy in tumored mice using an in vivo 212Pb/212Bi generator.

OBJECTIVE: Pretargeting is the concept that combines optimal delivery of the antibody and rapid capture and elimination of the radioactivity. In this study, we evaluated the potential of antibody pretargeting to enable the tumor-targeting (212)Pb for in vivo generation of (212)Bi for alpha particle radiotherapy. METHODS: The (212)Pb/(212)Bi chelate of DOTA-biotin, as well as their gamma-emitting analogues, (203)Pb and (205)Bi, was prepared and characterized. The radiolabeled compounds were injected in animals for evaluation of tumor targeting and normal tissue uptake and retention. In the pretargeting protocol, injection of 400 microg of NR-LU-10 antibody-streptavidin conjugate was given at t = 0 h, then 100 microg of N-acetyl-galatosamine-biotin clearing agent was injected at t = 20-24 h; finally, 1 microg of (212)Pb/(212)Bi-DOTA-biotin was injected 6 h later. RESULTS: Both (203)Pb and (205)Bi-DOTA-biotin were stable for at least 4 days in the different challenging solutions including PBS, 10 mM DTPA and serum. Contrary to its gamma-emitting analogues, radiolabeled (212)Pb-DOTA-biotin was not stable. There was greater than 30% of free (212)Bi released 4 h after (212)Pb-labeled DOTA-biotin. The results of pretargeting protocol of (203)Pb and (205)Bi-DOTA-biotin showed that the tumor target reached 20% injected dose (ID)/g at 4 h postinjection and remained high for 5 days. The %ID/g in the whole blood and other nontarget organs was low after administration of labeled (203)Pb and (205)Bi-DOTA-biotin similar to the biodistribution of labeled DOTA-biotin alone. In the animals administered (212)Pb-DOTA-biotin, radioactivity in nontarget organs was low except the kidneys. The %ID/g in the kidney for (212)Bi was 14.5 at 2 h, higher than (212)Pb, but dropped to about 6% ID/g by 4 h. However, tumor uptake for (212)Pb and (212)Bi was >25% ID/g at 1 h postinjection and remained so through 24 h. CONCLUSIONS: Antibody pretargeting system with Mab-streptavidin, clearing agent and DOTA-biotin provides the potential of (212)Bi for solid tumor radiotherapy despite the release of (212)Bi after (212)Pb decay. Dosimetry calculations resulted in tumor dose at 93 rad/muCi and ratios of tumor to marrow and kidney at 386:1 and 12:1, respectively.

Melanoma therapy via peptide-targeted {alpha}-radiation.

PURPOSE: The therapeutic efficacy of a unique melanoma-targeting peptide conjugated with an in vivo generated alpha-particle-emitting radionuclide was evaluated in the B16/F1 mouse melanoma animal model. alpha-Radiation is densely ionizing, resulting in high concentrations of destructive radicals and irreparable DNA double-strand breaks. This high linear energy transfer overcomes radiation-resistant tumor cells and oxygen effects resulting in potentially high therapeutic indices in tumors such as melanoma. EXPERIMENTAL DESIGN: The melanoma targeting peptide, 1,4,7,10-tetraazacyclodecane-1,4,7,10-tetraacetic acid (DOTA)-Re(Arg(11))CCMSH, was radiolabeled with (212)Pb, the parent of (212)Bi, which decays via alpha and beta decay. Biodistribution and therapy studies were done in the B16/F1 melanoma-bearing C57 mouse flank tumor model. RESULTS: (212)Pb[DOTA]-Re(Arg(11))CCMSH exhibited rapid tumor uptake and extended retention coupled with rapid whole body disappearance. Radiation dose delivered to the tumor was estimated to be 61 cGy/muCi (212)Pb administered. Treatment of melanoma-bearing mice with 50, 100, and 200 muCi of (212)Pb[DOTA]-Re(Arg(11))CCMSH extended their mean survival to 22, 28, and 49.8 days, respectively, compared with the 14.6-day mean survival of the placebo control group. Forty-five percent of the mice receiving 200 muCi doses survived the study disease-free. CONCLUSIONS: Treatment of B16/F1 murine melanoma-bearing mice with (212)Pb[DOTA]-Re(Arg(11))CCMSH significantly decreased tumor growth rates resulting in extended mean survival times, and in many cases, complete remission of disease. (212)Pb-DOTA-Re(Arg(11))CCMSH seems to be a very promising radiopharmaceutical for targeted radionuclide therapy of melanoma.

Pretargeted alpha emitting radioimmunotherapy using (213)Bi 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid-biotin.

PURPOSE: The use of an alpha emitter for radioimmunotherapy has potential advantages compared with beta emitters. When administered systemically optimal targeting of intact antibodies requires >24 h, therefore limiting the use of short-lived alpha emitters. This study investigated the biodistribution of bismuth-labeled biotin in A431 tumor-bearing mice pretargeted with antibody B3-streptavidin (B3-SA) and examined the therapeutic efficacy of the alpha emitter, (213)Bi-labeled biotin. EXPERIMENTAL DESIGN: Biotinidase-resistant 7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid (DOTA)-biotin was radiolabeled with (205,206)Bi or (213)Bi. Treatment of tumor-bearing mice began by administration of B3-SA (400 micro g) to target the tumor sites for 24 h. Then, an agent containing biotin and galactose groups was used to clear the conjugate from the circulation. Four h later, bismuth-radiolabeled DOTA-biotin was given, and biodistribution or therapy was evaluated. Dose escalation treatment from 3.7-74 MBq was performed, and the effects on tumors of different sizes were investigated. Tumor growth, complete blood cell counts, toxicity, and survival were monitored. RESULTS: Radiolabeled biotin cleared rapidly. Rapid tumor uptake resulted in much higher tumor:nontumor targeting ratios than achieved with the directly labeled monoclonal antibody. Dose escalation revealed that 74 MBq caused acute death of mice, whereas 0.37-37 MBq doses inhibited tumor growth and prolonged survival significantly. Evidence of mild hematological toxicity was noted. At therapeutically effective doses renal toxicity was observed. CONCLUSIONS: (213)Bi-DOTA-biotin, directed by the Pretarget method to tumor-targeted B3-SA, showed a therapeutic effect, although the therapeutic index was low. The source of the toxicity was most likely related to the renal toxicity.

In vivo evaluation of pretargeted 64Cu for tumor imaging and therapy.

UNLABELLED: Pretargeting involves administration of a tumor-targeting monoclonal antibody (mAb) covalently linked to a molecule having a high-affinity binding site for a rapidly distributed radiolabeled effector molecule. The aim of this study was to compare pretargeting to a conventionally labeled antibody for tumor targeting of the intermediate-lived radionuclide (64)Cu, which has shown promise for PET imaging and radioimmunotherapy of cancer. METHODS: DOTA-biotin (where DOTA is 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid) and the intact immunoconjugate DOTA-NR-LU-10 were labeled to high specific activities with (64)Cu, and the serum stabilities and target binding capabilities of each agent were assayed in vitro. Nude mice bearing SW1222 human colorectal carcinoma xenografts were administered (64)Cu-DOTA-biotin, with and without pretreatment with the mAb-streptavidin conjugate NR-LU-10/SA and the synthetic clearing agent Biotin-GalNAc(16), or injected with (64)Cu-DOTA-NR-LU-10. Biodistributions of both agents were obtained from 5 min to 48 h after injection. RESULTS: Both (64)Cu-DOTA-biotin and (64)Cu-DOTA-NR-LU-10 were 100% stable in serum in vitro. (64)Cu-DOTA-biotin exhibited >98% specific binding to immobilized streptavidin, whereas the immunoreactivity of (64)Cu-DOTA-NR-LU-10 averaged nearly 80%. Biodistributions in SW1222-bearing mice showed that NR-LU-10/SA-pretargeted (64)Cu-DOTA-biotin attained a peak tumor uptake of 18.9 percentage injected dose per gram (%ID/g) at 1 h, with concomitant rapid disappearance from blood and renal excretion. In the absence of pretargeting, (64)Cu-DOTA-biotin had very similar biodistribution and clearance properties, except with extremely low nonspecific tumor uptake. In contrast, (64)Cu-DOTA-NR-LU-10 reached 80.3 %ID/g in tumor tissue, after 48 h, whereas blood clearance was considerably slower than pretargeted (64)Cu-DOTA-biotin. Comparison of the time-activity curves for tumor uptake and blood clearance of pretargeted (64)Cu and the (64)Cu-labeled antibody revealed that the maximum tumor accumulations of radioactivity were similar for each agent, 17.9 percentage injected activity per gram (%IA/g) and 20.7 %IA/g, respectively. However, the tumor-to-blood ratio of areas under the curves was 14 times higher for pretargeted (64)Cu-DOTA-biotin because of the substantial increase in blood clearance of the small effector molecule. CONCLUSION: The extremely rapid tumor uptake and blood clearance of pretargeted (64)Cu-DOTA-biotin should afford markedly superior PET imaging contrast and therapeutic efficacy, compared with conventionally labeled (64)Cu-DOTA-NR-LU-10. Further comparison of the therapeutic efficacy, toxicity, and dosimetry of these 2 agents is warranted.

High-dose 166Ho-DOTMP in myeloablative treatment of multiple myeloma: pharmacokinetics, biodistribution, and absorbed dose estimation.

UNLABELLED: Thirty-two patients with multiple myeloma were treated with high doses of 166Ho-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethylene-phosphonic acid (DOTMP) and were a subset of patients enrolled in a multicenter phase I/II dose escalation myeloablative trial. 166Ho with beta-emission (half-life, 26.8 h; beta-particle energies, 1.85 MeV [51%] and 1.77 MeV [48%]; gamma-photons, 80.6 keV [6.6%] and 1.38 MeV [0.9%]) was complexed to DOTMP, a macrocyclic tetraphosphonate. Pharmacokinetics, dosimetry, and biodistribution were studied. METHODS: Patients were treated at escalating dose levels of 20, 30, and 40 Gy to the bone marrow in combination with high-dose melphalan, with or without total-body irradiation, to evaluate toxicity and efficacy. After infusion with 1,110 MBq (30 mCi) of 166Ho-DOTMP for evaluation of biodistribution and dosimetry calculation, patients received the calculated amount of radioactivity for therapy in a single administration based on estimated dose calculations. RESULTS: Thirty-two patients participated in the study and were then treated. The average amount of administered radioactivity was 74.3 GBq (2,007 mCi) (range, 21.5-147.5 GBq [581-3,987 mCi]) of 166Ho-DOTMP. CONCLUSION: 166Ho-DOTMP has physical and pharmacokinetic characteristics compatible with high-dose myeloablative treatment of multiple myeloma.