A Multispecific Anti-CD40 DARPin Construct Induces Tumor-Selective CD40 Activation and Tumor Regression.

The CD40 receptor is an attractive target for cancer immunotherapy. Although a modest pharmacodynamic effect is seen in patients following administration of CD40-targeting monoclonal antibodies (mAb), the doses that could be safely administered do not result in a meaningful clinical response, most likely due to the limited therapeutic window associated with systemic CD40 activation. To overcome this issue, we developed a multispecific DARPin construct, α-FAPxCD40, which has conditional activity at the site of disease. α-FAPxCD40 activation of CD40 depends on binding to fibroblast activation protein (FAP), a cell-surface protease overexpressed in the stroma of solid tumors. In vitro studies demonstrated that α-FAPxCD40 potently activates human antigen-presenting cells in the presence, but not in the absence, of FAP-positive cells. After intravenous injection, a murine surrogate construct (α-mFAPxCD40) accumulated in FAP-positive tumors, elicited rejection of 88% of these tumors, and induced memory antitumor immunity. Importantly, in contrast to the mouse anti-CD40 tested in parallel, the in vivo antitumor activity of α-mFAPxCD40 was associated neither with elevated blood cytokines nor with hepatotoxicity, both of which contribute to the clinical dose-limiting toxicities of several CD40 mAb. This study demonstrates that α-(m)FAPxCD40 engages CD40 in an FAP-restricted manner, leading to tumor eradication without signs of peripheral toxicity. This distinct preclinical profile suggests that a favorable therapeutic index may be achieved in humans. It further supports the development of α-FAPxCD40, currently tested in a first-in-human clinical study in patients with solid tumors (NCT05098405).

Receptor FGFRL1 does not promote cell proliferation but induces cell adhesion.

Fibroblast growth factor receptor (FGFR)-like protein 1 (FGFRL1) is the most recently discovered member of the FGFR family. Owing to the fact that it interacts with FGF ligands, but lacks the intracellular tyrosine kinase domain, several researchers have speculated that it may function as a decoy receptor and exert a negative effect on cell proliferation. In this study, we performed overexpression experiments with TetOn­inducible cell clones and downregulation experiments with siRNA oligonucleotides, and found that FGFRL1 had absolutely no effect on cell growth and proliferation. Likewise, we did not observe any influence of FGFRL1 on ERK1/2 activation and on the phosphorylation of 250 other signaling proteins analyzed by the Kinexus antibody microarray. On the other hand, with bacterial petri dishes, we observed a clear effect of FGFRL1 on cell adhesion during the initial hours after cell seeding. Our results suggest that FGFRL1 is a cell adhesion protein similar to the nectins rather than a signaling receptor similar to FGFR1-FGFR4.

Detection of ligand-induced CNTF receptor dimers in living cells by fluorescence cross correlation spectroscopy.

Ciliary neurotrophic factor (CNTF) signals via a receptor complex consisting of the specific CNTF receptor (CNTFR) and two promiscuous signal transducers, gp130 and leukemia inhibitory factor receptor (LIFR). Whereas earlier studies suggested that the signaling complex is a hexamer, more recent analyses strongly support a tetrameric structure. However, all studies so far analyzed the stoichiometry of the CNTF receptor complex in vitro and not in the context of living cells. We generated and expressed in mammalian cells acyl carrier protein-tagged versions of both CNTF and CNTFR. After labeling CNTF and CNTFR with different dyes we analyzed their diffusion behavior at the cell surface. Fluorescence (cross) correlation spectroscopy (FCS/FCCS) measurements reveal that CNTFR diffuses with a diffusion constant of about 2 x 10(-9) cm(2) s(-1) independent of whether CNTF is bound or not. FCS and FCCS measurements detect the formation of receptor complexes containing at least two CNTFs and CNTFRs. In addition, we measured Förster-type fluorescence resonance energy transfer between two differently labeled CNTFs within a receptor complex indicating a distance of 5-7 nm between the two. These findings are not consistent with a tetrameric structure of the CNTFR complex suggesting that either hexamers and or even higher-order structures (e.g. an octamer containing two tetramers) are formed.