Benefit of Later-Time-Point PET Imaging of HER3 Expression Using Optimized Radiocobalt-Labeled Affibody Molecules.

HER3-binding affibody molecules are a promising format for visualization of HER3 expression. Cobalt-55, a positron-emitting isotope, with a half-life of 17.5 h, allows for next-day imaging. We investigated the influence of the charge of the radiocobalt-chelator complex on the biodistribution of anti-HER3 affibody molecule (HE)-Z and compared the best radiocobalt-labeled variant with a recently optimized gallium-labeled variant. Affibody conjugates (HE)-Z-X (X = NOTA, NODAGA, DOTA, DOTAGA) were labeled with [Co]Co (surrogate for Co). Affinity measurements, binding specificity and cellular processing were studied in two HER3-expressing cancer cell lines. Biodistribution was studied 3 and 24 h post-injection (pi) in mice with HER3-expressing BxPC-3 xenografts and compared to [Ga]Ga-(HE)-Z-NODAGA. Micro-single-photon emission tomography/computed tomography (microSPECT/CT) and micro-positron emission tomography/computed tomography (microPET/CT) imaging was performed 3 and 24 h pi. Stably labeled conjugates bound to HER3 with subnanomolar affinity. [Co]Co-(HE)-Z-DOTA had the best tumor retention and a significantly lower concentration in blood than other conjugates, leading to superior tumor-to-blood and tumor-to-liver ratios 24 h pi. Compared to [Ga]Ga-(HE)-Z-NODAGA 3 h pi, [Co]Co-(HE)-Z-DOTA provided superior imaging contrast in liver 24 h pi. Concluding, the composition and charge of the [Co]Co-chelator complex influenced the uptake in tumors and normal tissue. [Co]Co-(HE)-Z-DOTA provided the best imaging properties among the cobalt-labeled conjugates. Delayed imaging of HER3 expression with [Co]Co-(HE)-Z-DOTA improved imaging contrast compared to early-time-point imaging with [Ga]Ga-(HE)-Z-NODAGA.