(175ae) “Transport Cocktails” of Alpha-Particle Radionuclide-Antibody Conjugates for Solid Tumors

Introduction: For 2024 in the US, at least 340,000 patients with advanced solid tumors (primary and/or metastatic) of the five most frequent cancers will die [1]. The promise of targeted alpha-particle radionuclide therapeutics (TAT) for solid tumors, which are refractory to standard therapies, is evident by the numerous clinical trials, most of which are currently evaluating alpha-particle radionuclide-antibody conjugates [2]. However, treated patients ultimately relapse. This is largely due to the inability to uniformly expose all malignant cells within solid tumors to alpha-particle radiation at sufficient levels to cause cell death. We have discovered a unique intervention that improves the therapeutic outcomes of alpha-particle radionuclide-antibody conjugates against established, soft-tissue solid tumors, without increasing the total administered activity.

Alpha-particle radiation is a potent, tumor agnostic therapy but only travels 4-5 cell diameters within tissue. Established (i.e. large, vascularized) solid tumors are particularly challenging: cells in deep tumor regions far from vasculature often do not receive sufficient doses of therapeutics injected in the blood. And the more strongly an antibody binds the less deep it penetrates into tumors (this is described by the binding site barrier effect [3, 4]), resulting in fewer cancer cells/tumor regions effectively being irradiated and killed.

We have developed a novel approach of “transport cocktails” of antibodies that more uniformly spread alpha-particle radiation within solid tumors and can, thus, improve the efficacy of any systemically injected targeting radionuclide-antibody conjugate. Alpha-particles (α-particles) are high-energy, short-range particles (travelling in tissue up to 40-80 µm) emitted from radionuclides, that physically break DNA as they traverse the cell nucleus. The inability to repair this DNA damage is the reason that α-particles [5] are impervious to resistance [6-8]. However, cells not being directly hit by alpha-particles will not be killed.

Our unique delivery strategy uniformly distributes α-particles within large solid tumors by simultaneously delivering the same α-particle emitter by different antibodies, each killing a different region of the tumor: (1) a non-targetingradiolabeled-antibody that upon tumor uptake penetrates the deeper parts of tumors where targeting-antibodies do not reach (Figure 1), and (2) a separately administered, targeting radiolabeled-antibody irradiating the tumor perivascular regions from where the non-targeting antibodies clear too fast, since they do not bind/adhere to cells and/or the tumor microenvironment to delay their clearance.

Our cocktail is different from previously reported cocktails of alpha-particle radionuclide-antibody conjugates, whereeach and every one of the antibodies in the cocktail are chosen to bind to a (different) receptor/marker expressed on the surface of the same cancer cells comprising the tumor [9]. The idea behind these cocktails with both antibodies binding to a different marker, is to collectively increase the radioactivity delivered per cancer cell, when none of the targeted markers is overexpressed by cancer cells. Our approach addresses the heterogeneous irradiation of solid tumors caused by the targeting/binding property of radionuclide-antibody conjugates.

Methods: The efficacy of the alpha-particle emitter actinium-225 delivered by our carrier-cocktails was assessed in vitro and on mice with ectopic xenografts of BT474 breast cancer, HEPG-2 hepatic cancer, and/or BxPC-3 pancreatic cancer. Cells/tumors were chosen to express different levels (from high-to-moderate to low) of HER1 or HER2, employed as model antibody-targeted markers.

Results: Figure 1 shows that the non-targeting antibodies (in blue) penetrate into the deep parts of spheroids that are used as surrogates of solid tumors’ avascular regions. Therefore, “transport cocktails” of, separate, targeting and non-targeting antibodies for the delivery of α-particle radiotherapy, collectively, irradiate and can kill all parts of tumors. Figure 2 shows that “transport cocktails” of radiolabeled antibodies (purple and/or green lines) inhibit tumor growth better and significantly improve survival of mice with solid tumors expressing different levels of the targeted marker, compared to the corresponding targeted radionuclide-antibody conjugates alone (red and/or orange lines), at same total injected activities. We employed tumors expressing the HER2 or the HER1 receptor targeted by Trastuzumab and/or Cetuximab, respectively. In all studies shown in Figure 2, the injected activity was equally split between the targeting and the non-targeting antibodies.

Conclusions: We have developed a “transport-oriented” delivery strategy using cocktails of separate radionuclide-antibody conjugates to collectively enable uniform irradiation of solid tumors by alpha-particles. Independent of tumor origin, type of targeted marker and/or expression level(s) of the targeted markers, we demonstrate improved tumor growth inhibition and survival with minimal toxicities for the “transport cocktails” compared to each type of radionuclide-antibody conjugate alone at same total injected activities. These findings demonstrate the potential of our “transport cocktails” to significantly enhance the therapeutic efficacy of existing alpha-particle radionuclide-antibody treatments, which are already evaluated in clinical trials.

References

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