Researchers report two dual-targeted GRPR/FAP heterodimeric radiopharmaceuticals, htdPP-01 and htdPP-02, built by combining the GRPR radioantagonist RM2 with a patented FAP-binding eFAP scaffold. The lead tracer, indium-111-labeled htdPP-02, stayed more than 51% intact in murine serum at 24 hours…
Investigators have built two heterodimeric radiopharmaceuticals that engage the gastrin-releasing peptide receptor GRPR and fibroblast activation protein FAP in a single molecule, and the lead compound, ¹¹¹In In-htdPP-02 , combines a sharp improvement in metabolic stability with preserved nanomolar binding to both targets and favorable uptake in preclinical breast tumor models. The compounds, htdPP-01 and htdPP-02 , are the first to combine the GRPR-targeting radioantagonist RM2 with a patented FAP-binding eFAP scaffold as single heterodimeric radioligands. The findings were reported in August 2026 and appear in the European Journal of Pharmaceutical Sciences, the official journal of the European Federation for Pharmaceutical Sciences.
The stability result frames the advance. At 24 hours in murine serum, both heterodimers remained more than 51% intact, while the RM2 monomer ¹¹¹In In-RM2 was only 12% intact. In vivo, ¹¹¹In In-htdPP-02 produced higher and more sustained uptake in T47-D breast tumor xenografts than ¹¹¹In In-RM2, and prolonged retention in HT-1080.huFAP xenografts compared with the FAP-targeted comparator ¹¹¹In In-FAPI-46 .
The design rationale is tumor heterogeneity. No single receptor marks every cell in a breast tumor. RM2 has shown promise for breast cancer imaging, but limited metabolic stability has hampered its in vivo performance, and a tracer aimed at one target will miss receptor-negative disease. The heterodimeric strategy attempts to engage tumor epithelial cells and the surrounding stroma with one pharmacokinetic entity.
The two heterodimers share the same architecture: a GRPR-binding module derived from RM2 and a FAP-binding module derived from the eFAP scaffold, combined into one radioligand. RM2 is a bombesin-based radioantagonist developed for peptide receptor imaging; it binds GRPR, a G protein-coupled receptor overexpressed on a substantial fraction of breast cancers. The eFAP scaffold is a patented, engineered FAP-binding moiety. FAP is a serine protease expressed by cancer-associated fibroblasts in the stroma of many solid tumors and has become one of the most studied targets in oncologic imaging.
The investigators synthesized two variants, htdPP-01 and htdPP-02, and characterized both in vitro, radiolabeling each with indium-111 and obtaining radiochemical yields and purities greater than 97% for both. They then selected htdPP-02 for in vivo evaluation based on its in vitro profile. htdPP-01 was not evaluated in animals.
The choice of RM2 as the peptide module is deliberate but carries a known cost. Peptides based on bombesin are degraded by serum peptidases, which shortens bioavailability and raises background signal. The serum stability measurements quantify that vulnerability: at the 24-hour time point, the heterodimers remained more than four times more intact than the RM2 monomer. Conjugation to the eFAP scaffold plausibly shields protease-sensitive bonds, although the structural basis of that effect is not established in the available findings. What the data establish is that fusing the two functions did not merely preserve the molecule; it stabilized the peptide half of it.
The in vitro characterization covered the properties that determine whether a radiotracer can move toward imaging, and the endpoint list ran from radiochemistry to tissue-level confirmation:
Both heterodimers exceeded 97% radiochemical yield and purity after labeling with indium-111, a SPECT radionuclide whose roughly 2.8 day physical half-life is well matched to 24-hour imaging protocols. Both compounds were hydrophilic, with logD 7.4 values of -2.51 ± 0.03 and -3.02 ± 0.04. Negative logD values at physiologic pH indicate hydrophilic character, which in peptide radiopharmaceuticals is generally associated with low nonspecific background uptake and predominantly renal clearance, at the cost of potentially faster washout from target tissue.
Both heterodimers retained nanomolar binding affinity for GRPR and for FAP. That is the central in vitro result: two targeting domains, each with its own binding surface, can be assembled into one molecule without either losing its interaction. Cell uptake experiments showed target-specific uptake comparable to the corresponding monomeric references, ¹¹¹In In-RM2 for the GRPR side and ¹¹¹In In-FAPI-46 for the FAP side.
Stability was measured in two environments. In phosphate-buffered saline, both heterodimers were more than 95% intact at 24 hours, indicating that neither the peptide bonds nor the radiolabel degrade spontaneously in buffer. In murine serum, the biologically relevant matrix, both remained more than 51% intact at the same time point, versus 12% for ¹¹¹In In-RM2. That gap is the quantitative case for the heterodimeric design.
The in vivo evaluation used two xenograft models to interrogate the two halves of the design separately. T47-D is a human breast cancer cell line with GRPR expression, used to examine the tumor cell side of the dual targeting. HT-1080.huFAP is a fibrosarcoma line engineered to express human FAP, used to examine the stroma side. MicroSPECT/CT imaging was performed at 24 hours after injection, the same time point used for the stability measurements.
In T47-D tumors, ¹¹¹In In-htdPP-02 showed higher and more sustained uptake than ¹¹¹In In-RM2, indicating that the stability advantage translated into improved tumor accumulation rather than being lost to altered pharmacokinetics. In HT-1080.huFAP tumors, the heterodimer showed prolonged retention compared with ¹¹¹In In-FAPI-46, evidence that the FAP-binding half of the molecule remained functional in vivo. Low background accumulation was observed, consistent with the hydrophilic character of the compound.
Two further assays tie the imaging signal to the intended targets. Ex vivo autoradiography confirmed target-specific binding in both xenograft models. Immunohistochemistry demonstrated extensive FAP immunoreactivity in T47-D xenografts, the mechanistically significant finding: the breast cancer model is not a pure GRPR model. FAP-bearing stroma is present within the same tumors imaged through GRPR. That coexistence of both targets in one tumor is the biological premise of the strategy. Whether the immunoreactivity reflects human FAP, mouse FAP, or both is not established; in xenografts, stromal fibroblasts are typically host-derived.
GRPR and FAP occupy different compartments of the tumor. GRPR is a receptor for gastrin-releasing peptide, a bombesin-like neuropeptide, and is overexpressed on a subset of breast cancer cells, where signaling through it can drive proliferation. It is an epithelial target. FAP is a serine protease expressed on cancer-associated fibroblasts, the activated cells that build the desmoplastic stroma around many breast tumors, and is largely absent from normal adult tissues. It is a stromal target.
The two markers are complementary in coverage. GRPR expression is heterogeneous across and within breast tumors and can shift with disease progression and treatment. FAP is abundant in stroma-rich tumors, including subtypes where epithelial receptor density is low. A single-target tracer images only the compartment that expresses its target. A dual-targeted tracer is designed to capture both, so a tumor that is negative for one target can still be detected through the other.
The heterodimeric format has a specific advantage over a simple mixture of two tracers. With a cocktail, each tracer has its own pharmacokinetics, clearance, and metabolism, so the two signals must be acquired and interpreted separately. In a heterodimer, both binding functions travel, distribute, and clear as one molecule, and the readout is a single integrated signal. The trade-off is molecular complexity: a larger conjugate can lose affinity, adopt unfavorable geometry, or clear too slowly. In this case affinity survived on both targets and serum stability improved, but the design risk is real and must be checked empirically for every new pair of targets.
For radiopharmaceutical chemists, the study is a worked example of how to build a dual-targeted peptide agent and what to measure while doing it. The endpoint list is instructive: purity, lipophilicity, affinity on each target, cell uptake and internalization, stability in buffer and serum, then imaging, autoradiography, and immunohistochemistry. The greater than 97% radiochemical yields and purities show that the labeling chemistry can be driven near completion, a practical point because residual unlabeled precursor competes for binding and adds background.
For clinicians, the question is whether a GRPR/FAP tracer will detect lesions that single-target tracers miss in breast cancer patients. That has not been shown. But the design addresses a recognized failure mode of receptor imaging: receptor-negative disease. If dual targeting increases the fraction of lesions detected or improves tumor-to-background ratios, it would give molecular imaging a more complete picture of disease burden than either agent alone.
For the supply chain, the strategy carries a constraint that will shape any translation effort. The eFAP scaffold is patented, and the identity of the patent holder is not disclosed in the available findings. Anyone seeking to reproduce the compounds, scale them under good manufacturing practice, or run clinical trials will need to resolve that intellectual property question. The compounds are heterodimeric conjugates, so production requires
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