Prostate-specific membrane antigen (PSMA) has been identified as a target for the development of theranostic agents. In our current work, we describe the design and synthesis of novel N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-(S)-L-lysine (DCL) urea-based PSMA inhibitors with a chlorine-substituted aromatic fragment at the lysine ε-nitrogen atom, a dipeptide including two phenylalanine residues in the L-configuration as the peptide fragment of the linker, and 3- or 4-(tributylstannyl)benzoic acid as a prosthetic group in their structures for radiolabeling. The standard compounds [¹²⁷I]PSMA-m-IB and [¹²⁷I]PSMA-p-IB for comparative and characterization studies were first synthesized using two alternative synthetic approaches. An important advantage of the alternative synthetic approach, in which the prosthetic group (NHS-activated esters of compounds) is first conjugated with the polypeptide sequence followed by replacement of the Sn(Bu)₃ group with radioiodine, is that the radionuclide is introduced in the final step of synthesis, thereby minimizing operating time with iodine-123 during the radiolabeling process. The obtained DCL urea-based PSMA inhibitors were radiolabeled with iodine-123. The radiolabeling optimization results showed that the radiochemical yield of [¹²³I]PSMA-p-IB was higher than that of [¹²³I]PSMA-m-IB, which were 74.9 ± 1.0% and 49.4 ± 1.2%, respectively. The radiochemical purity of [¹²³I]PSMA-p-IB after purification was greater than 99.50%. The initial preclinical evaluation of [¹²³I]PSMA-p-IB demonstrated a considerable affinity and specific binding to PC-3 PIP (PSMA-expressing cells) in vitro. The in vivo biodistribution of this new radioligand [¹²³I]PSMA-p-IB showed less accumulation than [¹⁷⁷Lu]Lu-PSMA-617 in several normal organs (liver, kidney, and bone). These results warrant further preclinical development, including toxicology evaluation and experiments in tumor-bearing mice.

Radionuclide molecular imaging of epidermal growth factor receptor (EGFR) expression might permit the selection of patients for EGFR-targeting therapies. Designed ankyrin repeat protein (DARPin) E01 with a high affinity to the ectodomain III of the EGFR is a possible EGFR imaging probe. The goal of this study was to evaluate the potential of radiolabeled DARPin E01 for in vivo imaging of EGFR. DARPin E01 containing the (HE)₃-tag was site-specifically labeled with a residualizing ^99mTc (using ^99mTc]Tc(CO)₃). Two methods providing non-residualizing ¹²³I labels, direct electrophilic radioiodination and indirect radioiodination using [¹²³I]I-para-iodobenzoate (PIB), were tested. [^99mTc]Tc-(HE)₃-E01 and [¹²³I]I-(HE)₃-E01-PIB preserved specific binding to EGFR-expressing cells and affinity in the single-digit nanomolar range. Direct labeling with ¹²³I resulted in a substantial loss of binding. In vitro cellular processing studies showed that both [^99mTc]Tc-(HE)₃-E01 and [¹²³I]I-(HE)₃-E01-PIB had rapid binding and relatively slow internalization. Evaluation of [^99mTc]Tc-(HE)₃-E01 biodistribution in normal CD1 mice showed that its hepatic uptake was non-saturable, suggesting that this tracer does not bind to murine EGFR. A side-by-side comparison of biodistribution and tumor targeting of [^99mTc]Tc-(HE)₃-E01 and [¹²³I]I-(HE)₃-E01-PIB was performed in Nu/j mice bearing EGFR-positive A-431 and EGFR-negative Ramos human cancer xenografts. Both radiolabeled DARPins demonstrated EGFR-specific tumor uptake. However, [¹²³I]I-(HE)₃-E01-PIB had appreciably lower uptake in normal organs compared to [^99mTc]Tc-(HE)₃-E01, which provided significantly (p < 0.05) higher tumor-to-organ ratios. Gamma-camera imaging confirmed that [¹²³I]I-(HE)₃-E01-PIB demonstrated a higher imaging contrast in preclinical models than [^99mTc]Tc-(HE)₃-E01. In conclusion, DARPin (HE)₃-E01 labeled using a non-residualizing [¹²³I]I-para-iodobenzoate (PIB) label is the preferred radiotracer for in vivo imaging of EGFR expression in cancer.

Prostate-specific membrane antigen (PSMA) has been identified as a target for the development of theranostic agents. In our current work, we describe the design and synthesis of novel N-[N-[(S)-1,3-dicarboxypropyl]carbamoyl]-(S)-L-lysine (DCL) urea-based PSMA inhibitors with a chlorine-substituted aromatic fragment at the lysine ε-nitrogen atom, a dipeptide as peptide fragment of the linker, including two phenylalanine residues in the L-configuration, and 3- or 4-(tributylstannyl)benzoic acid as prosthetic group in their structures for radiolabeling. The standard compounds ([127I]PSMA-m-IB and [127I]PSMA-p-IB) for comparative and characterization studies were firstly synthesized using two alternative synthetic approaches. An important advantage of the alternative synthetic approach in which the prosthetic group (NHS-activated esters of compounds) is first conjugated with the polypeptide sequence, followed by replacement of the Sn(Bu)3 group with radioiodine, is that the radionuclide is introduced in the final step of synthesis, thereby minimizing operating time with iodine-123 during the radiolabeling process. The obtained DCL urea-based PSMA inhibitors were radiolabeled with iodine-123. The radiolabeling optimization results showed that the radiochemical yield of [123I]PSMA-p-IB was higher than those of [123I]PSMA-m-IB, which were 74.9± 1.0% and 49.4 ± 1.2%, respectively. The radiochemical purity of [123I]PSMA-p-IB after purification was greater than 99.50%. The initial preclinical evaluation of [123I]PSMA-p-IB demonstrated a considerable affinity and specific binding to PC-3 PIP (PSMA-expressing cells) in vitro. The in vivo biodistribution of this new radioligand [123I]PSMA-p-IB showed less accumulation than [177Lu]Lu-PSMA-617 in several normal organs (liver, kidney and bone). These results warrants further preclinical development including toxicology evaluation and experiments in tumor-bearing mice.