68Ga-Labeled Maleimide for Blood Pool and Lymph PET Imaging through Covalent Bonding to Serum Albumin In Vivo

Development and Evaluation of DOTA-FAPI-Maleimide as a Novel Radiotracer for Tumor Theranostic with Extended Circulation

This study aimed to evaluate a novel albumin-binding strategy for addressing the challenge of insufficient tumor retention of fibroblast activation protein inhibitors (FAPIs). Maleimide, a molecule capable of covalent binding to free thiol groups, was modified to conjugate with FAPI-04 in order to enhance its binding to endogenous albumin, resulting in an extended blood circulation half-life and increased tumor uptake. DOTA-FAPI-maleimide was prepared and radiolabeled with Ga-68 and Lu-177, followed by cellular assays, pharmacokinetic analysis, PET/CT, and SPECT/CT imaging to assess the probe distribution in various tumor-bearing models. Radiolabeling of the modified probe was successfully achieved with a radiochemical yield of over 99% and remained stable for 144 h. Cellular assays showed that the ligand concentration required for 50% inhibition of the probe was 1.20 ± 0.31 nM, and the Kd was 0.70 ± 0.07 nM with a Bmax of 7.94 ± 0.16 fmol/cell, indicative of higher specificity and affinity of DOTA-FAPI-maleimide compared to other FAPI-04 variants. In addition, DOTA-FAPI-maleimide exhibited a persistent blood clearance half-life of 7.11 ± 0.34 h. PET/CT images showed a tumor uptake of 2.20 ± 0.44%ID/g at 0.5 h p.i., with a tumor/muscle ratio of 5.64 in HT-1080-FAP tumor-bearing models. SPECT/CT images demonstrated long-lasting tumor retention. At 24 h p.i., the tumor uptake of [177Lu]Lu-DOTA-FAPI-maleimide reached 5.04 ± 1.67%ID/g, with stable tumor retention of 3.40 ± 1.95%ID/g after 4 days p.i. In conclusion, we developed and evaluated the thiol group-attaching strategy, which significantly extended the circulation and tumor retention of the adapted FAPI tracer. We envision its potential application for clinical cancer theranostics.

Fundamental evaluation regarding the relationship between albumin-binding and tumor accumulation of PSMA-targeting radioligands

OBJECTIVE

The marked success of prostate-specific membrane antigen (PSMA)-targeting radioligands with albumin binder (ALB) is attributed to the improvement of blood retention and tumor accumulation. [111In]In-PNT-DA1, our PSMA-targeting radioligand with ALB, also achieved improved tumor accumulation due to its prolonged blood retention. Although the advantage of ALBs is related to their reversible binding to albumin, the relationship between albumin-binding and tumor accumulation of PSMA-targeting radioligands remains unclear because of the lack of information about radioligands with stronger albumin-binding than ALBs. In this study, we designed and synthesized [111In]In-PNT-DM-HSA, a new radioligand that consists of a PSMA-targeting radioligand covalently bound to albumin. The pharmacokinetics of [111In]In-PNT-DM-HSA was compared with those of [111In]In-PNT-DA1 and [111In]In-PSMA-617, a non-ALB-conjugated radioligand, to evaluate the relationship between albumin-binding and tumor accumulation.

METHOD

The [111In]In-PNT-DM-HSA was prepared by incubation of [111In]In-PNT-DM, a PSMA-targeting radioligand including a maleimide group, and human serum albumin (HSA). The ability of [111In]In-PNT-DM-HSA was evaluated by in vitro assays. A biodistribution study using LNCaP tumor-bearing mice was conducted to compare the pharmacokinetics of [111In]In-PNT-DM-HSA, [111In]In-PNT-DA1, and [111In]In-PSMA-617.

RESULTS

The [111In]In-PNT-DM-HSA was obtained at a favorable radiochemical yield and high radiochemical purity. In vitro assays revealed that [111In]In-PNT-DM-HSA had fundamental characteristics as a PSMA-targeting radioligand interacting with albumin covalently. In a biodistribution study, [111In]In-PNT-DM-HSA and [111In]In-PNT-DA1 showed higher blood retention than [111In]In-PSMA-617. On the other hand, the tumor accumulation of [111In]In-PNT-DA1 was much higher than [111In]In-PNT-DM-HSA and [111In]In-PSMA-617.

CONCLUSIONS

These results indicate that the moderate reversible binding of ALB with albumin, not covalent binding, may play a critical role in enhancing the tumor accumulation of PSMA-targeting radioligands.

Albumin-based nanosystem for dual-modality imaging-guided chem-phototherapy against immune-cold triple-negative breast cancer

Triple-negative breast cancer is a highly aggressive and metastatic tumor; diagnosing it in the early stages is still difficult, and the prognosis for conventional radio-chemotherapy and immunotreatment is not promising due to cancer's immunosuppressive microenvironment. The utilization of protein-based nanosystem has proven to be effective in delivering agents with limited adverse effects, yet the combination of diagnosis and treatment remains a difficult challenge. This research took advantage of natural albumin and organic molecules to construct a self-assemble core-shell nanostructure combining with superparamagnetic iron oxide nanocrystals and heptamethine cyanine dye IR780 through non-covalent interactions. This nanocomposite successfully decreased the transverse relaxation time of the magnetic resonance hydrogen nucleus, resulting in outstanding T2 imaging, as well as emitting near-infrared II fluorescence, thereby the resulting dual-modality imaging tool was applied to improve diagnostic competency. It is noteworthy that the nanocomposites exhibited impressive enzyme-like catalytic and photothermal capabilities, resulting in a successful activation of the immune system to efficiently suppress distant metastatic lesions in vivo. Consequently, this nano-drug-based therapy could be an advantageous asset in reinforcing the immune system and hindering the growth and reappearance of the immune-cold breast cancer.

Preclinical PET Imaging and Toxicity Study of a 68Ga-Functionalized Polymeric Cardiac Blood Pool Agent

Cardiac blood pool imaging is currently performed almost exclusively with 99mTc-based compounds and SPECT/CT imaging. Using a generator-based PET radioisotope has a few advantages, including not needing nuclear reactors to produce it, obtaining better resolution in humans, and potentially reducing the radiation dose to the patient. When the shortlived radioisotope 68Ga is used, it can be applied repeatedly on the same day—for example, for the detection of bleeding. Our objective was to prepare and evaluate a long-circulating polymer functionalized with gallium for its biodistribution, toxicity, and dosimetric properties. A 500 kDa hyperbranched polyglycerol was conjugated to the chelator NOTA and radiolabeled rapidly at room temperature with 68Ga. It was then injected intravenously into a rat, and gated imaging allowed us to easily observe wall motion and cardiac contractility, confirming the suitability of this radiopharmaceutical for cardiac blood pool imaging. Internal radiation dose calculations showed that the radiation doses that patients would receive from the PET agent would be 2.5× lower than those from the 99mTc agent. A complete 14-day toxicology study in rats concluded that there were no gross pathology findings, changes in body or organ weights, or histopathological events. This radioactive-metal-functionalized polymer might be a suitable non-toxic agent to advance for clinical application.