PSMA-reactive NB7 single domain antibody fragment: A potential scaffold for developing prostate cancer theranostics.

INTRODUCTION: Single domain antibody fragments (sdAbs) are an appealing scaffold for radiopharmaceutical development due to their small size (~15 kDa), high solubility, high stability, and excellent tumor penetration. Previously, we developed NB7 sdAb, which has very high affinity for an epitope on PSMA that is different from those targeted by small molecule PSMA inhibitors. Herein, we evaluated NB7 after radioiodination using [*I]SGMIB (1,3,4-isomer) and iso-[*I]SGMIB (1,3,5-isomer), as well as their 211 At-labeled analogues.

METHODS: [*I]SGMIB, iso-[*I]SGMIB, [211 At]SAGMB, and iso-[211 At]SAGMB conjugates of NB7 sdAb were synthesized and their binding affinity, cell uptake and internalization were assessed in PSMA+ PC3 PIP and PSMA- PC3 flu cells. Biodistribution studies were performed in mice bearing PSMA+ PC3 PIP xenografts. First, a single-label experiment evaluated the tissue distribution of a NB7 bearing a His6 -tag (NB7H6) and labeled with iso-[125 I]SGMIB. Three paired-label experiments then were performed to compare: a) NB7 labeled using [*I]SGMIB and iso-[*I]SGMIB, b) 131 I- vs 211 At-labeled NB7 conjugates and c) [125 I]SGMIB-NB7H6 to the small molecule PSMA inhibitor [131 I]YF2.

RESULTS: All NB7 radioconjugates bound specifically to PSMA with dissociation constants, Kd , in the low nM range (1.4-6.4 nM). An initial biodistribution study demonstrated good tumor uptake for iso-[125 I]SGMIB-NB7H6 (7.2 ± 1.5 % ID/g at 1 h) and no deleterious effect of the His6 -tag on renal activity levels, which declined to 3.1 ± 1.1 % ID/g by 4 h. Paired-label biodistribution found no distinction between the two SGMIB isomer NB7 conjugates with the [131 I]SGMIB-NB7-to-iso-[125 I]SGMIB-NB7 tumor uptake ratios not significantly different from unity: 1.06 ± 0.08 at 1 h, 1.04 ± 0.12 at 4 h, and 1.07 ± 0.09 at 24 h. Both isomer conjugates cleared rapidly from normal tissues and exhibited very low uptake in thyroid, lacrimal and salivary glands. Paired-label biodistribution of [131 I]SGMIB-NB7H6 and [211 At]SAGMB-NB7H6 demonstrated similar tumor uptake and kidney clearance for the two radioconjugates. However, levels of 211 At in thyroid, stomach, salivary and lacrimal glands were significantly higher (P < 0.05) that those for 131 I suggesting greater dehalogenation for [211 At]SAGMB-NB7H6. Finally, co-administration of [125 I]SGMIB-NB7H6 and [131 I]YF2 demonstrated good tumor uptake for both with considerably more rapid renal clearance for the NB7 radioconjugate.

CONCLUSION: NB7 radioconjugates exhibited good accumulation in PSMA-positive xenografts with rapid clearance from kidney and other normal tissues. We conclude that NB7 is a potentially useful scaffold for developing PSMA-targeted theranostics with different characteristics than current small molecule and antibody-based approaches.