FAP Radioligand Linker Optimization Improves Tumor Dose and Tumor-to-Healthy Organ Ratios in 4T1 Syngeneic Model

Fibroblast activation protein (FAP) has attracted considerable attention as a possible target for the radiotherapy of solid tumors. Unfortunately, initial efforts to treat solid tumors with FAP-targeted radionuclides have yielded only modest clinical responses, suggesting that further improvements in the molecular design of FAP-targeted radiopharmaceutical therapies (RPT) are warranted. In this study, we report several advances on the previously described FAP6 radioligand that increase tumor retention and accelerate healthy tissue clearance. Seven FAP6 derivatives with different linkers or albumin binders were synthesized, radiolabeled, and investigated for their effects on binding and cellular uptake. The radioligands were then characterized in 4T1 tumor-bearing Balb/c mice using both single-photon emission computed tomography (SPECT) and ex vivo biodistribution analyses to identify the conjugate with the best tumor retention and tumor-to-healthy organ ratios. The results reveal an optimized FAP6 radioligand that exhibits efficacy and safety properties that potentially justify its translation into the clinic.

Innovative Design Strategies Advance Biomedical Applications of Phthalocyanines

Owing to their long absorption wavelengths, high molar absorptivity, and tunable photosensitivity, phthalocyanines have been widely used in photodynamic therapy (PDT). However, phthalocyanines still face the drawbacks of poor targeting, "always-on" photosensitizing properties, and unsatisfactory therapeutic efficiency, which limit their wide applications in biomedical fields. Thus, new design strategies such as modification of targeting molecules, formation of nanoparticles, and activating photosensitizers are developed to improve the above defects. Notably, recent studies have shown that novel phthalocyanines are not only used in fluorescence imaging and PDT, but also in photoacoustic imaging, photothermal imaging, sonodynamic therapy, and photothermal therapy. This review focuses on recent design strategies, applications in biomedicine, and clinical development of phthalocyanines, providing ideas and references for the design and application of phthalocyanine, so as to promote their future transformation into clinical applications.

Targeted cancer phototherapy using phthalocyanine-anticancer drug conjugates

Phototherapy, the use of light to selectively ablate cancerous tissue, is a compelling prospect. Phototherapy is divided into two major domains: photodynamic and photothermal, whereby photosensitizer irradiation generates reactive oxygen species or heat, respectively, to disrupt the cancer microenvironment. Phthalocyanines (Pcs) are prominent phototherapeutics due to their desirable optical properties and structural versatility. Targeting of Pc photosensitizers historically relied on the enhanced permeation and retention effect, but the weak specificity engendered by this approach has hindered bench-to-clinic translation. To improve specificity, antibody and peptide active-targeting groups have been employed to some effect. An alternative targeting method exploits the binding of anticancer drugs to direct the photosensitizer close to essential cellular components, allowing for precise, synergistic phototherapy. This Perspective explores the use of Pc-drug conjugates as targeted anticancer phototherapeutic systems with examples of Pc-platin, Pc-kinase, and Pc-anthracycline conjugates discussed in detail.

Preparation of porphyrin and phthalocyanine conjugates for biomedical applications

The aim of this review is to offer a succinct overview of the main synthetic techniques used in the preparation of conjugates containing porphyrins, including chlorins and bacteriochlorins and phthalocyanines for biomedical applications and photodynamic therapy (PDT). To illustrate the conjugation techniques representative examples from the literature will be presented.

Molecular recognition of fibroblast activation protein for diagnostic and therapeutic applications

Fibroblast activation protein (FAP) is a non-classical serine protease expressed predominantly in conditions accompanied by tissue remodeling, particularly cancer. Due to its plasma membrane localization, FAP represents a promising molecular target for tumor imaging and treatment. The unique enzymatic activity of FAP facilitates development of diagnostic and therapeutic tools based on molecular recognition of FAP by substrates and small-molecule inhibitors, in addition to conventional antibody-based strategies. In this review, we provide background on the pathophysiological role of FAP and discuss its potential for diagnostic and therapeutic applications. Furthermore, we present a detailed analysis of the structural patterns crucial for substrate and inhibitor recognition by the FAP active site and determinants of selectivity over the related proteases dipeptidyl peptidase IV and prolyl endopeptidase. We also review published data on targeting of the tumor microenvironment with FAP antibodies, FAP-targeted prodrugs, activity-based probes and small-molecule inhibitors. We describe use of a recently developed, selective FAP inhibitor with low-nanomolar potency in inhibitor-based targeting strategies including synthetic antibody mimetics based on hydrophilic polymers and inhibitor conjugates for PET imaging. In conclusion, recent advances in understanding of the molecular structure and function of FAP have significantly contributed to the development of several tools with potential for translation into clinical practice.