Novel Preparation Methods of (52)Mn for ImmunoPET Imaging

Abstract

(52)Mn (t1/2 = 5.59 d, β(+) = 29.6%, Eβave = 0.24 MeV) shows promise in positron emission tomography (PET) and in dual-modality manganese-enhanced magnetic resonance imaging (MEMRI) applications including neural tractography, stem cell tracking, and biological toxicity studies. The extension to bioconjugate application requires high-specific-activity (52)Mn in a state suitable for macromolecule labeling. To that end a (52)Mn production, purification, and labeling system is presented, and its applicability in preclinical, macromolecule PET is shown using the conjugate (52)Mn-DOTA-TRC105. (52)Mn is produced by 60 μA, 16 MeV proton irradiation of natural chromium metal pressed into a silver disc support. Radiochemical separation proceeds by strong anion exchange chromatography of the dissolved Cr target, employing a semiorganic mobile phase, 97:3 (v:v) ethanol:HCl (11 M, aqueous). The method is 62 ± 14% efficient (n = 7) in (52)Mn recovery, leading to a separation factor from Cr of (1.6 ± 1.0) × 10(6) (n = 4), and an average effective specific activity of 0.8 GBq/μmol (n = 4) in titration against DOTA. (52)Mn-DOTA-TRC105 conjugation and labeling demonstrate the potential for chelation applications. In vivo images acquired using PET/CT in mice bearing 4T1 xenograft tumors are presented. Peak tumor uptake is 18.7 ± 2.7%ID/g at 24 h post injection and ex vivo (52)Mn biodistribution validates the in vivo PET data. Free (52)Mn(2+) (as chloride or acetate) is used as a control in additional mice to evaluate the nontargeted biodistribution in the tumor model.

Figure 1

99.999% pure chromium pellet imbedded in silver, direct-jet watercooled on the rear target face, before (A) and after (B) irradiation by 60 μA of 16 MeV protons for 1 h.

Figure 2

Serial maximum intensity projection (MIP) PET images of mice injected with ⁵²Mn-DOTA-TRC105 and ⁵²MnCl₂. Significant thyroid accumulation in the ⁵²MnCl₂ images contrasting the lack of uptake in the ⁵²Mn-DOTA-TRC105 images indicates highly stable DOTA chelation of ⁵²Mn²⁺ even at late time-points. Note: H, Heart; L, Liver; K, Kidneys; T, Tumor; Th, Thyroid.

Figure 3

Time activity curves (TACs) acquired from ROI analysis of PET images obtained in subjects injected with ⁵²Mn-DOTA-TRC105 (n = 3) and with Free ⁵²Mn (n = 4). Error bars represent the standard deviation, and are displayed one-sided for visual clarity. The Heart/ Blood TAC uses an ROI drawn over the left ventricle and does not differentiate myocardium from the blood pool.

Figure 4

Ex vivo ⁵²Mn-DOTA-TRC105 (n = 3) and ⁵²MnCl₂/⁵²Mn-acetate (n = 4) biodistribution data obtained following the last PET time point. Animals were sacrificed, and tissue samples were isolated, washed with saline, dried, weighed, and gamma counted.