In-111DAC Is a Novel Technique To Image Multiple Myeloma.

-Background: An increase in the Vitamin B12 (cobalamin) serum transport protein, transcobalamin II (TCII), has been noted in myeloma (MM) patients (Carmel, Blood. 1978.51(6):1057). Uptake of B12 bound to TCII is regulated by transcobalamin II receptors (TCII-R). Labeled thymidine studies have shown, in vitro, that TCII-R are up-regulated in proliferating malignant cells and correlate with increased incorporation of ³H-thymidine and DNA synthesis (

Indium-111diethylenetriaminepentaacetate adenosylcobalamin (In-111DAC) is a novel radionuclide labeled B12 conjugate that has shown preferential uptake in malignant tissue compared to normal tissue via clinical gamma camera imaging of nude mice with sarcoma xenografts and human patients with various solid tumors (Collins, J Nuc Med, 1997.38(5):717 and Mayo Clin Proc, 2000.75(6):568). Nearly all the human tumors were visualized with In-111DAC and a correlation between increased In-111 uptake and higher invasiveness of the tumor at biopsy was observed. We questioned whether In-111DAC could be utilized to image MM, in vivo in murine models.

Methods: In accordance with the Institutional Animal Care and Use Committee, we assessed In-111DAC as an imaging agent for MM in mice with subcutaneous (SQ) MM tumors and mice with systemic MM. 7 week old CB17/scid mice received total body irradiation 150cGy before 1x10⁷ myeloma cells (KAS-6/1-gift of Dr. D.F. Jelinek) were injected SQ between the scapulae. Once tumors reached at least 5mm, both tumor bearing mice and saline control mice without tumor were injected intraperitoneally (IP) with 400 μCi In-111DAC. Mice with SQ tumors were injected with 400 μCi of free In-111 for comparison. Gamma camera images at 15h were followed by CT/SPECT (Single photon emission tomography) images at 18h after IP injection. To evaluate In-111DAC imaging in mice with systemic MM, 7 week old bg-nu-xid mice were injected via tail vein with 1x10⁷ myeloma cells (5TGM1-gift of B.O. Oyajobi). Animals with late stage myeloma (day 23–28) and saline control animals were injected with In-111DAC 400 μCi, and were compared to control animals with late MM, injected with 400 μCi of free In-111. Animals underwent gamma camera and CT/SPECT imaging as in the time-frame above. All animals were sacrificed. Organs and tumors were harvested and weighed. Radioactivity was measured in a NaI scintillation well counter and recorded as cpm/mg.

Results: Images and radioactivity measurements in the SQ MM model (KAS-6/1) demonstrated excellent uptake of In-111DAC within the tumor compared to normal tissue and controls. In the 5TGM1 systemic MM model, the spleen is a known sanctuary of MM and displayed especially strong uptake of In-111DAC compared to normal tissue.

Conclusion: We report the first murine MM models to be imaged with In-111DAC. Our study affirms that In-111DAC is an excellent imaging agent for MM. In-111DAC imaging could be utilized clinically to complement standard staging studies to assess MM tumor burden in patients at time of diagnosis, relapse, or to assess treatment response; especially in patients with non-secretory disease. Moreover, our study strongly suggests the rationale to exploit TCII-R as a target for MM therapy.