Transport-driven engineering of liposomes for delivery of α-particle radiotherapy to solid tumors: effect on inhibition of tumor progression and onset delay of spontaneous metastases

Low-dose temozolomide selectively increases glioblastoma's vascular permeability, tumor microenvironment penetration and the killing potential of systemic actinium-225 α-particle dendrimer-radioconjugates improving treatment efficacy

PURPOSE

The poor prognosis of glioblastoma is mostly due to the relatively low tumor vascular permeability to therapeutics, the tumor's vicinity to the brain, that limits treatment aggressiveness, and/or drug resistance.

METHODS

In this study, the efficacy of systemically injected actinium-225 dendrimer-radioconjugates was evaluated in an immune-competent orthotopic GL261-C57BL/6 mouse model after administration of low-dose, standard-of-care temozolomide, that selectively increased the tumor vascular permeability to dendrimer-radioconjugates. Alpha-particles' short range in tissue combined with the dendrimers' selective uptake by glioblastomas, could limit the irradiation of the neighboring brain, while the complex double-strand DNA breaks caused by α-particles were expected to be largely impervious to resistance by cancer cells.

RESULTS

On mice bearing 9.7 ± 5.7mm3 brain tumors, at activities that did not cause long-term (11-months) toxicities, dendrimer-radioconjugates, that were systemically-administered 24-hours after injection of temozolomide, significantly improved survival compared to dendrimer-radioconjugates alone (44 vs. 39 days mean survival, p = 0.0017) and/or compared to temozolomide alone and/or to non-treated animals (31 and 30 days, p < 0.001). This was attributed to: (1) the noteworthy increase (by 33%) in tumor absorbed doses delivered by dendrimer-radioconjugates when injected after chemotherapy, without altering normal organ (including the brain's) dosimetry; (2) the potentially deeper tumor penetration of dendrimer-radioconjugates, suggested by the enhanced dendrimer penetration within GL261-spheroids, employed as model tumor-avascular regions; and/or (3) the formation of a more lethal cocktail when both modalities acted on same cancer cells.

CONCLUSIONS

This study demonstrates the potential and safety of actinium-225 dendrimer-radioconjugates as a systemic α-particle radiotherapy for glioblastoma enhanced by low-dose temozolomide.

MIRD Pamphlet No. 31: MIRDcell V4-Artificial Intelligence Tools to Formulate Optimized Radiopharmaceutical Cocktails for Therapy

Radiopharmaceutical cocktails have been developed over the years to treat cancer. Cocktails of agents are attractive because 1 radiopharmaceutical is unlikely to have the desired therapeutic effect because of nonuniform uptake by the targeted cells. Therefore, multiple radiopharmaceuticals targeting different receptors on a cell is warranted. However, past implementations in vivo have not met with convincing results because of the absence of optimization strategies. Here we present artificial intelligence (AI) tools housed in a new version of our software platform, MIRDcell V4, that optimize a cocktail of radiopharmaceuticals by minimizing the total disintegrations needed to achieve a given surviving fraction (SF) of tumor cells. Methods: AI tools are developed within MIRDcell V4 using an optimizer based on the sequential least-squares programming algorithm. The algorithm determines the molar activities for each drug in the cocktail that minimize the total disintegrations required to achieve a specified SF. Tools are provided for populations of cells that do not cross-irradiate (e.g., circulating or disseminated tumor cells) and for multicellular clusters (e.g., micrometastases). The tools were tested using model data, flow cytometry data for suspensions of single cells labeled with fluorochrome-labeled antibodies, and 3-dimensional spatiotemporal kinetics in spheroids for fluorochrome-loaded liposomes. Results: Experimental binding distributions of 4 211At-antibodies were considered for treating suspensions of MDA-MB-231 human breast cancer cells. A 2-drug combination reduced the number of 211At decays required by a factor of 1.6 relative to the best single antibody. In another study, 2 radiopharmaceuticals radiolabeled with 195mPt were each distributed lognormally in a hypothetical multicellular cluster. Here, the 2-drug combination required 1.7-fold fewer decays than did either drug alone. Finally, 2 225Ac-labeled drugs that provide different radial distributions within a spheroid require about one half of the disintegrations required by the best single agent. Conclusion: The MIRDcell AI tools determine optimized drug combinations and corresponding molar activities needed to achieve a given SF. This approach could be used to analyze a sample of cells obtained from cell culture, animal, or patient to predict the best combination of drugs for maximum therapeutic effect with the least total disintegrations.

Surface-functionalised polymeric nanoparticles for breast cancer treatment: processes and advances

The World Health Organization (WHO) reported that of all the non-communicable diseases, cancer is considered the second cause of death worldwide. This has driven the big pharma companies to prioritise anticancer products in their pipeline. In addition, research has focused on exploration of new anticancer molecules and design of suitable dosage forms to achieve effective drug delivery to the tumour site. Nanotechnology is a valuable tool to build nano delivery systems with controlled and targeted drug release properties. Nanoparticles can be fabricated by robust, scalable and economic techniques using various polymers. Moreover, specific functional groups can be introduced to the surface of nanoparticles enabling targeting to a specific tissue; besides, they exhibit versatile drug release patterns according to the rate of polymer degradation. This review outlines the processes and advances in surface functionalisation of nanoparticles employed for treatment of breast cancer. The therapeutic molecules, the polymers used to fabricate nanoparticles, the techniques used to prepare the nanoparticles have been reviewed with a focus on the processes employed to functionalise these nanoparticles with suitable ligands to target different types of breast cancer.

Combined, yet separate: cocktails of carriers (not drugs) for actinium-225 α-particle therapy of solid tumors expressing moderate-to-low levels of targetable markers

Alpha-particle radionuclide-antibody conjugates are being clinically evaluated against solid tumors even when they moderately express the targeted markers. At this limit of lower tumor-absorbed doses, to maintain efficacy, the few(er) intratumorally delivered alpha-particles need to traverse/hit as many different cancer cells as possible. We complement antibody-radioconjugate therapies with a separate nanocarrier delivering a fraction of the same total injected radioactivity to tumor regions geographically different than those affected by targeting antibodies; these carrier-cocktails collectively distribute the alpha-particle emitters better.

METHODS

The efficacy of actinium-225 delivered by our carrier-cocktails was assessed in vitro and on mice with orthotopic MDA-MB-436 and/or MDA-MB-231 triple-negative breast cancers and/or an ectopic BxPC3 pancreatic cancer. Cells/tumors were chosen to express low-to-moderate levels of HER1, as model antibody-targeted marker.

RESULTS

Independent of cell line, antibody-radioconjugates were most lethal on cell monolayers. On spheroids, with radii greater than alpha-particles' range, carrier-cocktails improved killing efficacy (p < 0.0500). Treatment with carrier-cocktails decreased the MDA-MB-436 and MDA-MB-231 orthotopic tumor volumes by 73.7% and 72.1%, respectively, relative to treatment with antibody-radioconjugates alone, at same total injected radioactivity; these carrier-cocktails completely eliminated formation of spontaneous metastases vs. 50% and 25% elimination in mice treated with antibody-radioconjugates alone. In BxPC3 tumor-bearing mice, carrier-cocktails increased the median survival to 25-26 days (in male-female animals) vs. 20-21 days of mice treated with antibody-radioconjugates alone (vs. 17 days for non-treated animals). Survival with carrier-cocktail radiotherapy was further prolonged by pre-injecting low-dose, standard-of-care, gemcitabine (p = 0.0390).

CONCLUSION

Tumor-agnostic carrier-cocktails significantly enhance the therapeutic efficacy of existing alpha-particle radionuclide-antibody treatments.

Development of 225Ac-doped biocompatible nanoparticles for targeted alpha therapy

Targeted alpha therapy (TAT) relies on chemical affinity or active targeting using radioimmunoconjugates as strategies to deliver α-emitting radionuclides to cancerous tissue. These strategies can be affected by transmetalation of the parent radionuclide by competing ions in vivo and the bond-breaking recoil energy of decay daughters. The retention of α-emitting radionuclides and the dose delivered to cancer cells are influenced by these processes. Encapsulating α-emitting radionuclides within nanoparticles can help overcome many of these challenges. Poly(lactic-co-glycolic acid) (PLGA) nanoparticles are a biodegradable and biocompatible delivery platform that has been used for drug delivery. In this study, PLGA nanoparticles are utilized for encapsulation and retention of actinium-225 ([225Ac]Ac3+). Encapsulation of [225Ac]Ac3+ within PLGA nanoparticles (Zave = 155.3 nm) was achieved by adapting a double-emulsion solvent evaporation method. The encapsulation efficiency was affected by both the solvent conditions and the chelation of [225Ac]Ac3+. Chelation of [225Ac]Ac3+ to a lipophilic 2,9-bis-lactam-1,10-phenanthroline ligand ([225Ac]AcBLPhen) significantly decreased its release (< 2%) and that of its decay daughters (< 50%) from PLGA nanoparticles. PLGA nanoparticles encapsulating [225Ac]AcBLPhen significantly increased the delivery of [225Ac]Ac3+ to murine (E0771) and human (MCF-7 and MDA-MB-231) breast cancer cells with a concomitant increase in cell death over free [225Ac]Ac3+ in solution. These results demonstrate that PLGA nanoparticles have potential as radionuclide delivery platforms for TAT to advance precision radiotherapy for cancer. In addition, this technology offers an alternative use for ligands with poor aqueous solubility, low stability, or low affinity, allowing them to be repurposed for TAT by encapsulation within PLGA nanoparticles.

Can current preclinical strategies for radiopharmaceutical development meet the needs of targeted alpha therapy?

Preclinical studies are essential for effectively evaluating TAT radiopharmaceuticals. Given the current suboptimal supply chain of these radionuclides, animal studies must be refined to produce the most translatable TAT agents with the greatest clinical potential. Vector design is pivotal, emphasizing harmonious physical and biological characteristics among the vector, target, and radionuclide. The scarcity of alpha-emitting radionuclides remains a significant consideration. Actinium-225 and lead-212 appear as the most readily available radionuclides at this stage. Available animal models for researchers encompass xenografts, allografts, and PDX (patient-derived xenograft) models. Emerging strategies for imaging alpha-emitters are also briefly explored. Ultimately, preclinical research must address two critical aspects: (1) offering valuable insights into balancing safety and efficacy, and (2) providing guidance on the optimal dosing of the TAT agent.

Recent advances in the development of 225Ac- and 211At-labeled radioligands for radiotheranostics

Radiotheranostics utilizes a set of radioligands incorporating diagnostic or therapeutic radionuclides to achieve both diagnosis and therapy. Imaging probes using diagnostic radionuclides have been used for systemic cancer imaging. Integration of therapeutic radionuclides into the imaging probes serves as potent agents for radionuclide therapy. Among them, targeted alpha therapy (TAT) is a promising next-generation cancer therapy. The α-particles emitted by the radioligands used in TAT result in a high linear energy transfer over a short range, inducing substantial damage to nearby cells surrounding the binding site. Therefore, the key to successful cancer treatment with minimal side effects by TAT depends on the selective delivery of radioligands to their targets. Recently, TAT agents targeting biomolecules highly expressed in various cancer cells, such as sodium/iodide symporter, norepinephrine transporter, somatostatin receptor, αvβ3 integrin, prostate-specific membrane antigen, fibroblast-activation protein, and human epidermal growth factor receptor 2 have been developed and have made remarkable progress toward clinical application. In this review, we focus on two radionuclides, 225Ac and 211At, which are expected to have a wide range of applications in TAT. We also introduce recent fundamental and clinical studies of radiopharmaceuticals labeled with these radionuclides.

Early Normal Tissue Effects and Bone Marrow Relative Biological Effectiveness for an Actinium 225-Labeled HER2/neu-Targeting Antibody

PURPOSE

In this study we determined the dose-independent relative biological effectiveness (RBE2) of bone marrow for an anti-HER2/neu antibody labeled with the alpha-particle emitter actinium 225 (225Ac). Hematologic toxicity is often a consequence of radiopharmaceutical therapy (RPT) administration, and dosimetric guidance to the bone marrow is required to limit toxicity.

METHODS AND MATERIALS

Female neu/N transgenic mice (MMTV-neu) were intravenously injected with 0 to 16.65 kBq of the alpha-particle emitter labeled antibody, 225Ac-DOTA-7.16.4, and euthanized at 1 to 9 days after treatment. Complete blood counts were performed. Femurs and tibias were collected, and bone marrow was isolated from 1 femur and tibia and counted for radioactivity. Contralateral intact femurs were fixed, decalcified, and assessed by histology. Marrow cellularity was the biologic endpoint selected for RBE2 determination. For the reference radiation, both femurs of the mice were photon irradiated with 0 to 5 Gy using a small animal radiation research platform.

RESULTS

Response as measured by cellularity for the alpha-particle emitter RPT (αRPT) RPT and the external beam radiation therapy were linear and linear quadratic, respectively, as a function of absorbed dose. The resulting dose-independent RBE2 for bone marrow was 6.

CONCLUSIONS

As αRPT gains prominence, preclinical studies evaluating RBE in vivo will be important in relating to human experience with beta-particle emitter RPT. Such normal tissue RBE evaluations will help mitigate unexpected toxicity in αRPT.

Predicting response of micrometastases with MIRDcell V3: proof of principle with 225Ac-DOTA encapsulating liposomes that produce different activity distributions in tumor spheroids

PURPOSE

The spatial distribution of radiopharmaceuticals within multicellular clusters is known to have a significant effect on their biological response. Most therapeutic radiopharmaceuticals distribute nonuniformly in tissues which makes predicting responses of micrometastases challenging. The work presented here analyzes published temporally dependent nonuniform activity distributions within tumor spheroids treated with actinium-225-DOTA encapsulating liposomes (225Ac-liposomes) and uses these data in MIRDcell V3.11 to calculate absorbed dose distributions and predict biological response. The predicted responses are compared with experimental responses.

METHODS

Four types of liposomes were prepared having membranes with different combinations of release (R) and adhesion (A) properties. The combinations were R-A-, R-A+, R+A-, and R+A+. These afford different penetrating properties into tissue. The liposomes were loaded with either carboxyfluorescein diacetate succinimidyl ester (CFDA-SE) or 225Ac. MDA-MB-231 spheroids were treated with the CFDA-SE-liposomes, harvested at different times, and the time-integrated CFDA-SE concentration at each radial position within the spheroid was determined. This was translated into mean 225Ac decays/cell versus radial position, uploaded to MIRDcell, and the surviving fraction of cells in spherical multicellular clusters was simulated. The MIRDcell-predicted surviving fractions were compared with experimental fractional-outgrowths of the spheroids following treatment with 225Ac-liposomes.

RESULTS

The biological responses of the multicellular clusters treated with 225Ac-liposomes with physicochemical properties R+A+, R-A+, and R-A- were predicted by MIRDcell with statistically significant accuracy. The prediction for R+A- was not predicted accurately.

CONCLUSION

In most instances, MIRDcell predicts responses of spheroids treated with 225Ac-liposomes that result in different tissue-penetrating profiles of the delivered radionuclides.

Combination of Carriers with Complementary Intratumoral Microdistributions of Delivered -Particles May Realize the Promise for 225Ac in Large, Solid Tumors

α-particle radiotherapy has already been shown to be impervious to most resistance mechanisms. However, in established (i.e., large, vascularized) soft-tissue lesions, the diffusion-limited penetration depths of radiolabeled antibodies or nanocarriers (≤50-80 μm) combined with the short range of α-particles (4-5 cell diameters) may result in only partial tumor irradiation, potentially limiting treatment efficacy. To address this challenge, we combined carriers with complementary intratumoral microdistributions of the delivered α-particles. We used the α-particle generator ²²⁵Ac, and we combined a tumor-responsive liposome (which, on tumor uptake, releases into the interstitium a highly diffusing form of its radioactive payload [²²⁵Ac-DOTA], potentially penetrating the deeper parts of tumors where antibodies do not reach) with a separately administered, less-penetrating radiolabeled antibody (irradiating the tumor perivascular regions where liposome contents clear too quickly). Methods: In a murine model with orthotopic human epidermal growth factor receptor 2-positive BT474 breast cancer xenografts, the biodistributions of each carrier were evaluated, and the control of tumor growth was monitored after administration of the same total radioactivity of ²²⁵Ac delivered by the ²²⁵Ac-DOTA-encapsulating liposomes, by the ²²⁵Ac-DOTA-SCN--labeled trastuzumab, and by both carriers at equally split radioactivities. Results: Tumor growth was significantly more inhibited when the same total injected radioactivity was divided between the 2 separate carriers than when delivered by either of the carriers alone. The combined carriers enabled more uniform intratumoral microdistributions of α-particles, at a tumor dose that was lower than the dose delivered by the antibody alone. Conclusion: This strategy demonstrates that more uniform microdistributions of the delivered α-particles within established solid tumors improve efficacy even at lower tumor doses. Augmentation of antibody-targeted α-particle therapies with tumor-responsive liposomes may address partial tumor irradiation, improving therapeutic effects.

Two diverse carriers are better than one: A case study in α‐particle therapy for prostate specific membrane antigen‐expressing prostate cancers

Partial and/or heterogeneous irradiation of established (i.e., large, vascularized) tumors by α‐particles that exhibit only a 4–5 cell‐diameter range in tissue, limits the therapeutic effect, since regions not being hit by the high energy α‐particles are likely not to be killed. This study aims to mechanistically understand a delivery strategy to uniformly distribute α‐particles within established solid tumors by simultaneously delivering the same α‐particle emitter by two diverse carriers, each killing a different region of the tumor: (1) the cancer‐agnostic, but also tumor‐responsive, liposomes engineered to best irradiate tumor regions far from the vasculature, and (2) a separately administered, antibody, targeting any cancer‐cell's surface marker, to best irradiate the tumor perivascular regions. We demonstrate that on a prostate specific membrane antigen (PSMA)‐expressing prostate cancer xenograft mouse model, for the same total injected radioactivity of the α‐particle emitter Actinium‐225, any radioactivity split ratio between the two carriers resulted in better tumor growth inhibition compared to the tumor inhibition when the total radioactivity was delivered by any of the two carriers alone. This finding was due to more uniform tumor irradiation for the same total injected radioactivity. The killing efficacy was improved even though the tumor‐absorbed dose delivered by the combined carriers was lower than the tumor‐absorbed dose delivered by the antibody alone. Studies on spheroids with different receptor‐expression, used as surrogates of the tumors' avascular regions, demonstrated that our delivery strategy is valid even for as low as 1+ (ImmunoHistoChemistry score) PSMA‐levels. The findings presented herein may hold clinical promise for those established tumors not being effectively eradicated by current α‐particle radiotherapies.

Growth Inhibition of Triple-Negative Breast Cancer: The Role of Spatiotemporal Delivery of Neoadjuvant Doxorubicin and Cisplatin

Combinations of platinum-based compounds with doxorubicin in free and/or in liposomal form for improved safety are currently being evaluated in the neoadjuvant setting on patients with advanced triple-negative breast cancer (TNBC). However, TNBC may likely be driven by chemotherapy-resistant cells. Additionally, established TNBC tumors may also exhibit diffusion-limited transport, resulting in heterogeneous intratumoral delivery of the administered therapeutics; this limits therapeutic efficacy in vivo. We studied TNBC cells with variable chemosensitivities, in the absence (on monolayers) and presence (in 3D multicellular spheroids) of transport barriers; we compared the combined killing effect of free doxorubicin and free cisplatin to the killing effect (1) of conventional liposomal forms of the two chemotherapeutics, and (2) of tumor-responsive lipid nanoparticles (NP), specifically engineered to result in more uniform spatiotemporal microdistributions of the agents within solid tumors. This was enabled by the NP properties of interstitial release, cell binding/internalization, and/or adhesion to the tumors’ extracellular matrix. The synergistic cell kill by combinations of the agents (in all forms), compared to the killing effect of each agent alone, was validated on monolayers of cells. Especially for spheroids formed by cells exhibiting resistance to doxorubicin combination treatments with both agents in free and/or in tumor-responsive NP-forms were comparably effective; we not only observed greater inhibition of outgrowth compared to the single agent(s) but also compared to the conventional liposome forms of the combined agents. We correlated this finding to more uniform spatiotemporal microdistributions of agents by the tumor-responsive NP. Our study shows that combinations of NP with properties specifically optimized to improve the spatiotemporal uniformity of the delivery of their corresponding therapeutic cargo can improve treatment efficacy while keeping favorable safety profiles.