Chimeric antigen receptor (CAR) T-cell therapy has revolutionized the treatment landscape of hematologic malignancies but has demonstrated limited efficacy in solid tumors, mainly due to the complex and immunosuppressive tumor microenvironment (TME). Among the strategies to overcome this challenge, targeting the tumor stroma rather than the tumor cells themselves has gained increasing interest. In this context, fibroblast activation protein alpha (FAP), a cell surface protease overexpressed by cancer-associated fibroblasts, represents a promising target. With the aim of remodeling the TME, enhancing immune infiltration, and suppressing tumor growth, numerous FAP-directed CAR T-cell therapies have been developed in the last decade, leading to the clinical translation of two candidates. To improve the flexibility and safety profile of CAR T-cell therapies, several groups have designed more controllable and modular approaches, including adapter CAR T-cell systems, which enable on-demand activation of effector cells through the administration of an adapter molecule. In parallel, the development of FAP-targeted radiotracers, particularly FAP inhibitors (FAPIs), has enabled high-contrast imaging of solid tumors and introduced attractive opportunities for radioligand therapy. The convergence of these advances has given rise to immunotheranostic strategies that integrate CAR T-cell immunotherapy and radioligand delivery within a unified framework. This review traces the evolution of FAP-directed CAR T-cell strategies, from conventional designs to adapter-based and theranostic platforms, and examines how modular adapters bring immunotherapy and radioligand delivery together within a single immunotheranostic framework, across preclinical and clinical settings.

Chimeric antigen receptor (CAR) therapies are emerging as promising strategies, particularly for metastatic castration-resistant prostate cancer (PCa), as they can act independently of the androgen receptor axis. Adapter CAR-T cell platforms, such as the RevCAR system, offer precise therapeutic control and tumor targeting via small, rapidly eliminated tumor-specific adapters. To enable more convenient late-stage RevCAR-T therapy in PCa patients, allowing for discontinuous reverse target module (RevTM) infusion, we developed novel, larger IgG4-based RevTMs targeting prostate stem cell antigen (PSCA) and benchmarked them against previously described smaller adapter formats. Within the RevCAR system, PSCA-IgG4 RevTMs effectively mediated PCa killing at low effector-to-target ratios and low RevTM concentrations in a strictly antigen-dependent manner. Oncolytic activity was accompanied by a rapid and pronounced release of proinflammatory cytokines across a broad RevTM concentration range, which is particularly advantageous for immunologically cold PCa. Finally, anti-tumor activity was confirmed in a short-term mouse model. Preliminary PET studies further indicate slow blood elimination and tumor-specific accumulation of novel IgG4-RevTMs. Together, these data position PSCA-IgG4 RevTMs as promising candidates for stepwise RevCAR-T treatment in PCa, in which short-lived scFv-RevTMs are initially used to ensure a rapid safety switch, followed by larger IgG4-RevTMs once the risk profile is known.

Due to its overexpression in the tumor microenvironment of most solid malignancies, fibroblast activation protein (FAP) has emerged as an ideal target for (immuno)theranostic applications. The clinically tested UniCAR system represents a promising adapter CAR T-cell approach, in which CAR T-cell activity is regulated through the administration of target modules (TMs). Here, we report the first homodimeric FAP inhibitor (FAPI)-based TMs that enable efficient adapter CAR T-cell immunotherapy of FAP-positive tumors. The novel TMs consist of two UAMC-1110 FAPI moieties, the E5B9 UniCAR epitope, a suitable polyethylene glycol spacer to ensure epitope accessibility, and a functional group that allows for further TM functionalization for diagnostic and radiotherapeutic applications. Following the synthesis of the novel FAPI TMs, we evaluated their functionality using both in vitro and in vivo models. The FAPI TMs successfully redirect UniCAR T-cells and mediate potent lysis of FAP-positive cells in vitro and in an immunodeficient mouse model. In addition, we established a 3D heterospheroid model consisting of tumor cells expressing prostate stem cell antigen (PSCA) alongside FAP-positive stromal fibroblasts. We showed that simultaneous dual-targeting leads to enhanced UniCAR-mediated cytotoxicity. Notably, fibroblast killing is mediated by the release of PSCA from dying tumor cells and its subsequent binding to the surface of neighboring PSCA-negative fibroblasts, thereby making them susceptible to PSCA-directed targeting. Overall, our work not only summarizes the successful development of novel dimeric FAPI adapter molecules for immunotherapeutic applications in solid tumors but also provides novel insights into the targeting of PSCA-positive tumors.

The fibroblast activation protein alpha (FAPα) is overexpressed in the tumor microenvironment of most solid cancers and, in some cases, in cancer cells, making it an interesting target for theranostic applications. T-cells modified to express a chimeric antigen receptor (CAR) against FAP have recently been described. We previously established the UniCAR system, in which UniCAR T-cells can be repeatedly switched on and off via dosing with a bifunctional adaptor molecule, known as target module (TM). Here, we describe the first FAPI-based immunotheranostic UniCAR TMs (FAPI TMs), enabling both non-invasive molecular imaging and UniCAR T-cell immunotherapy. The FAPI TMs consist of the UAMC-1110 FAPI moiety, the NODA-GA chelator for copper-64 labeling, and the UniCAR epitope (E5B9). Following computational analyses, three FAPI TMs with polyethylene glycol (PEG) spacers of either four, twelve, or 24 units were synthesized. Although the three novel TMs specifically accumulate in FAP-positive tumors in xenograft mice, only the FAPI TMs with an extended spacer (PEG12 and PEG24) redirect UniCAR T-cells to FAP-positive target cells both in vitro and in an immunodeficient mouse model. In line with the computational studies, the E5B9 epitope is not accessible for binding when the PEG4-based FAPI TM is bound to FAP. Our work demonstrates that the length of the spacer in FAPI TMs is critical for the effective redirection of UniCAR T-cells to FAP-positive cells. Overall, our novel FAPI TMs may represent highly promising immunotheranostic tools for personalized non-invasive diagnostic imaging and immunotherapy of cancer patients.

The new era of living drugs has increased the demand for innovative ways to monitor their presence in vivo. We present a label-free sensor for sensitive detection of immunotherapeutic drugs in serum. We developed an extended gate sensor based on a single commercial field effect transistor (FET), reducing the cost of the overall device. The sensing component consists of an in-house fabricated chip of 32 gold electrodes on glass, which combined with the electronic multiplexing and readout modules, allows for simultaneous data acquisition. The sensor was utilized for detection of a bispecific molecule, called the Target Module (TM), a crucial component of the Universal CAR (UniCAR) system, a safer module of the adaptive CAR T-cell therapy. We utilized the sensor to screen TMs of different sizes through a versatile functionalized layer that resourcefully aims to minimize the Debye screening length effect. The detection of the TMs was achieved in buffer and serum at the femtomolar level, significantly below the limit of the standard colorimetric technique ELISA. Additionally, we utilized the same system to track the presence of the TMs in mouse serum over time. The recorded data showed an expected shorter half-life for the smaller TM (scFv TM 32 kDa) as compared to the bigger TM (IgG4 TM 112 kDa). These results were comparable with the radioactivity data circumventing the need for radioactive labelling. Overall, we demonstrate a promising application of low-cost, label-free, and accurate extended gate (EG)-FET biosensor for immunotherapeutic drug monitoring and dose adjustment.

Target modules (TMs), intermediate molecules required for UniCAR T-cell therapy, are promising molecules for immunotheranostic approaches. In the current work, we developed TMs containing a monomeric or dimeric form of the antagonist bombesin peptide (BBN2) and assessed their potential for diagnostic imaging using positron emission tomography (PET) as well as immunotherapy in combination with UniCAR T-cells to target and image GRPR expression in prostate cancer. Synthesized monomeric and dimeric BBN2 TMs retained binding to GRPR in vitro. Both BBN2 TMs specifically activated and redirected UniCAR T-cells to eradicate PC3 and LNCaP cancer cells with high efficiency and in a comparable manner. UniCAR T-cells retained a non-exhausted memory phenotype favorable to their persistence and fitness. The 68Ga-labeled BBN2 TMs showed proof-of-target towards GRPR in PC3 and LNCaP xenografts with similar uptake profiles for both BBN2 TMs in dynamic PET experiments. Clearance occurred exclusively through renal elimination. A tremendously increased in vivo metabolic stability of the BBN2 TMs was observed compared to their counterparts without E5B9. Both monomeric and dimeric BBN2 TMs represent novel and promising immunotheranostic tools for application in prostate cancer with exceptionally high in vivo metabolic stability.

Micrometastases are challenging to resect surgically and to detect with in vivo imaging. Immunotherapy is highly anticipated to revolutionize their treatment, but its overall efficacy still remains limited for solid tumors. Here, a 3D micrometastases model is developed to mimic key microenvironmental cues, enabling in vitro evaluation of chimeric antigen receptor (CAR) T cell immunotherapy. Prostate cancer that preferentially metastasizes to, e.g., liver or bone marrow, is utilized as a model. Hydrogel beads with an elastic modulus matching those of soft organs are used to support long-term culturing, immunostaining, and monitoring of the spheroids. As a biochemical cue, the impact of fibroblast activation protein (FAP), an emerging target in the tumor microenvironment, is investigated on prostate cancer spheroids and on the efficacy of CAR T cell therapy. The multi-spheroid model consists of prostate stem cell antigen (PSCA)-expressing prostate cancer cells and FAP-producing fibrosarcoma cells in varying ratios. The morphological features of the model are compared to clinical histopathology and metastatic murine model samples. Finally, CAR T cell trials demonstrate successful chemoattraction and infiltration through the hydrogel matrix, with a dual-targeting approach against FAP and PSCA antigens showing synergistic efficacy. This research provides invaluable insights for engineering 3D tumor models and modeling therapies targeting small metastatic or residual tumors, suggesting that co-targeting may be a more effective strategy to unlock the tumor microenvironment's suppression.

A novel approach designed to directly estimate microcanonical quantities from energy histograms is proposed, which enables the immediate systematic identification and classification of phase transitions in physical systems of any size by means of the recently introduced generalized microcanonical inflection-point analysis method. The application to the aggregation problem of GNNQQNY heptapeptides, for which the entire transition sequence is revealed, shows the power of this promising method.

Applying CAR T-cell therapy to treat solid tumors is especially challenging due to the immunosuppressive tumor microenvironment (TME). While our modular RevCAR system enhances the safety and controllability of CAR T-cell therapy, effectively targeting solid tumors remains difficult. Since PD-L1 is an immune checkpoint frequently upregulated by cancer cells and their microenvironment, it is a relevant target for solid tumors. Here, we introduce a novel PD-L1 RevTM capable of redirecting RevCAR T-cells to specifically target and kill PD-L1-expressing tumor cells, becoming activated and secreting pro-inflammatory cytokines. This is shown in vitro with monolayer and 3D models, including patient-derived cultures, and in vivo. Furthermore, we demonstrate in vitro and in vivo an AND-gated targeting of cells simultaneously expressing PD-L1 and another tumor-associated antigen by the Dual RevCAR system. Our findings suggest that RevCAR-mediated targeting of PD-L1 could be a promising therapeutic approach for modulating the TME and improving solid tumor treatment.