Targeted radionuclide therapy: proceedings of a joint workshop hosted by the National Cancer Institute and the Society of Nuclear Medicine and Molecular Imaging

National Cancer Institute support for targeted alpha-emitter therapy

BACKGROUND

Radiopharmaceutical targeted therapy (RPT) has been studied for decades; however, recent clinical trials demonstrating efficacy have helped renewed interest in the modality.

METHODS

This article reviews National Cancer Institute (NCI)'s support of RPT through communication via workshops and interest groups, through funding extramural programs in academia and small business, and through intramural research, including preclinical and clinical studies.

RESULTS

NCI has co-organized workshops and organized interest groups on RPT and RPT dosimetry to encourage the community and facilitate rigorous preclinical and clinical studies. NCI has been supporting RPT research through various mechanisms. Research has been funded through peer-reviewed NCI Research and Program Grants (RPG) and NCI Small Business Innovation Research (SBIR) Development Center, which funds small business-initiated projects, some of which have led to clinical trials. The NCI Cancer Therapy Evaluation Program (CTEP)'s Radiopharmaceutical Development Initiative supports RPT in NCI-funded clinical trials, including Imaging and Radiation Oncology Core (IROC) expertise in imaging QA and dosimetry procedures. Preclinical targeted a-emitter therapy (TAT) research at the NCI's intramural program is ongoing, building on foundational work dating back to the 1980s. Ongoing "bench-to-bedside" efforts leverage the unique infrastructure of the National Institutes of Health's (NIH) Clinical Center.

CONCLUSION

Given the great potential of RPT, our goal is to continue to encourage its development that will generate the high-quality evidence needed to bring this multidisciplinary treatment to patients.

Biodistribution and internal radiation dosimetry of a companion diagnostic radiopharmaceutical, [68Ga]PSMA-11, in subcutaneous prostate cancer xenograft model mice

[⁶⁸Ga]PSMA-11 is a prostate-specific membrane antigen (PSMA)-targeting radiopharmaceutical for diagnostic PET imaging. Its application can be extended to targeted radionuclide therapy (TRT). In this study, we characterize the biodistribution and pharmacokinetics of [⁶⁸Ga]PSMA-11 in PSMA-positive and negative (22Rv1 and PC3, respectively) tumor-bearing mice and subsequently estimated its internal radiation dosimetry via voxel-level dosimetry using a dedicated Monte Carlo simulation to evaluate the absorbed dose in the tumor directly. Consequently, this approach overcomes the drawbacks of the conventional organ-level (or phantom-based) method. The kidneys and urinary bladder both showed substantial accumulation of [⁶⁸Ga]PSMA-11 without exhibiting a washout phase during the study. For the tumor, a peak concentration of 4.5 ± 0.7 %ID/g occurred 90 min after [⁶⁸Ga]PSMA-11 injection. The voxel- and organ-level methods both determined that the highest absorbed dose occurred in the kidneys (0.209 ± 0.005 Gy/MBq and 0.492 ± 0.059 Gy/MBq, respectively). Using voxel-level dosimetry, the absorbed dose in the tumor was estimated as 0.024 ± 0.003 Gy/MBq. The biodistribution and pharmacokinetics of [⁶⁸Ga]PSMA-11 in various organs of subcutaneous prostate cancer xenograft model mice were consistent with reported data for prostate cancer patients. Therefore, our data supports the use of voxel-level dosimetry in TRT to deliver personalized dosimetry considering patient-specific heterogeneous tissue compositions and activity distributions.

Deep-dose: a voxel dose estimation method using deep convolutional neural network for personalized internal dosimetry

Personalized dosimetry with high accuracy is crucial owing to the growing interests in personalized medicine. The direct Monte Carlo simulation is considered as a state-of-art voxel-based dosimetry technique; however, it incurs an excessive computational cost and time. To overcome the limitations of the direct Monte Carlo approach, we propose using a deep convolutional neural network (CNN) for the voxel dose prediction. PET and CT image patches were used as inputs for the CNN with the given ground truth from direct Monte Carlo. The predicted voxel dose rate maps from the CNN were compared with the ground truth and dose rate maps generated voxel S-value (VSV) kernel convolution method, which is one of the common voxel-based dosimetry techniques. The CNN-based dose rate map agreed well with the ground truth with voxel dose rate errors of 2.54% ± 2.09%. The VSV kernel approach showed a voxel error of 9.97% ± 1.79%. In the whole-body dosimetry study, the average organ absorbed dose errors were 1.07%, 9.43%, and 34.22% for the CNN, VSV, and OLINDA/EXM dosimetry software, respectively. The proposed CNN-based dosimetry method showed improvements compared to the conventional dosimetry approaches and showed results comparable with that of the direct Monte Carlo simulation with significantly lower calculation time.

Advancing Targeted Radionuclide Therapy Through the National Cancer Institute's Small Business Innovation Research Pathway

The Small Business Innovation Research (SBIR) and Small Business Technology Transfer (STTR) programs of the National Cancer Institute (NCI) are congressionally mandated set-aside programs that provide research funding to for-profit small businesses for the development of innovative technologies and treatments that serve the public good. These two programs have an annual budget of $159 million (in 2017) and serve as the NCI's main engine of innovation for developing and commercializing cancer technologies. In collaboration with the NCI's Radiation Research Program, the NCI SBIR Development Center published in 2015-2017 three separate requests for proposals from small businesses for the development of systemic targeted radionuclide therapy (TRT) technologies to treat cancer. TRT combines a cytotoxic radioactive isotope with a molecularly targeted agent to produce an anticancer therapy capable of treating local or systemic disease. This article summarizes the NCI SBIR funding solicitations for the development of TRTs and the research proposals funded through them.

Future cancer research priorities in the USA: a Lancet Oncology Commission

We are in the midst of a technological revolution that is providing new insights into human biology and cancer. In this era of big data, we are amassing large amounts of information that is transforming how we approach cancer treatment and prevention. Enactment of the Cancer Moonshot within the 21st Century Cures Act in the USA arrived at a propitious moment in the advancement of knowledge, providing nearly US$2 billion of funding for cancer research and precision medicine. In 2016, the Blue Ribbon Panel (BRP) set out a roadmap of recommendations designed to exploit new advances in cancer diagnosis, prevention, and treatment. Those recommendations provided a high-level view of how to accelerate the conversion of new scientific discoveries into effective treatments and prevention for cancer. The US National Cancer Institute is already implementing some of those recommendations. As experts in the priority areas identified by the BRP, we bolster those recommendations to implement this important scientific roadmap. In this Commission, we examine the BRP recommendations in greater detail and expand the discussion to include additional priority areas, including surgical oncology, radiation oncology, imaging, health systems and health disparities, regulation and financing, population science, and oncopolicy. We prioritise areas of research in the USA that we believe would accelerate efforts to benefit patients with cancer. Finally, we hope the recommendations in this report will facilitate new international collaborations to further enhance global efforts in cancer control.

A Microdosimetric-Kinetic Model of Cell Killing by Irradiation from Permanently Incorporated Radionuclides

An expression for the surviving fraction of a replicating population of cells exposed to permanently incorporated radionuclide is derived from the microdosimetric-kinetic model. It includes dependency on total implant dose, linear energy transfer (LET), decay rate of the radionuclide, the repair rate of potentially lethal lesions in DNA and the volume doubling time of the target population. This is used to obtain an expression for the biologically effective dose ( BED_α/β) based on the minimum survival achieved by the implant that is equivalent to, and can be compared and combined with, the BED_α/β calculated for a fractionated course of radiation treatment. Approximate relationships are presented that are useful in the calculation of BED_α/β for alpha- or beta-emitting radionuclides with half-life significantly greater than, or nearly equal to, the approximately 1-h repair half-life of radiation-induced potentially lethal lesions.

Targeting angiogenesis for radioimmunotherapy with a 177Lu-labeled antibody

PURPOSE

Increased angiogenesis is a marker of aggressiveness in many cancers. Targeted radionuclide therapy of these cancers with angiogenesis-targeting agents may curtail this increased blood vessel formation and slow the growth of tumors, both primary and metastatic. CD105, or endoglin, has a primary role in angiogenesis in a number of cancers, making this a widely applicable target for targeted radioimmunotherapy.

METHODS

The anti-CD105 antibody, TRC105 (TRACON Pharmaceuticals), was conjugated with DTPA for radiolabeling with ¹⁷⁷Lu (t _1/2 6.65 days). Balb/c mice were implanted with 4T1 mammary carcinoma cells, and five study groups were used: ¹⁷⁷Lu only, TRC105 only, ¹⁷⁷Lu-DTPA-IgG (a nonspecific antibody), ¹⁷⁷Lu-DTPA-TRC105 low-dose, and ¹⁷⁷Lu-DTPA-TRC105 high-dose. Toxicity of the agent was monitored by body weight measurements and analysis of blood markers. Biodistribution studies of ¹⁷⁷Lu-DTPA-TRC105 were also performed at 1 and 7 days after injection. Ex vivo histology studies of various tissues were conducted at 1, 7, and 30 days after injection of high-dose ¹⁷⁷Lu-DTPA-TRC105.

RESULTS

Biodistribution studies indicated steady uptake of ¹⁷⁷Lu-DTPA-TRC105 in 4T1 tumors between 1 and 7 days after injection (14.3 ± 2.3%ID/g and 11.6 ± 6.1%ID/g, respectively; n = 3) and gradual clearance from other organs. Significant inhibition of tumor growth was observed in the high-dose group, with a corresponding significant increase in survival (p < 0.001, all groups). In most study groups (all except the nonspecific IgG group), the body weights of the mice did not decrease by more than 10%, indicating the safety of the injected agents. Serum alanine transaminase levels remained nearly constant indicating no damage to the liver (a primary clearance organ of the agent), and this was confirmed by ex vivo histological analyses.

CONCLUSION

¹⁷⁷Lu-DTPA-TRC105, when administered at a sufficient dose, is able to curtail tumor growth and provide a significant survival benefit without off-target toxicity. Thus, this targeted agent could be used in combination with other treatment options to slow tumor growth allowing the other agents to be more effective.

Targeted Radionuclide Therapy: An Evolution Toward Precision Cancer Treatment

OBJECTIVE

This article reviews recent developments in targeted radionuclide therapy (TRT) approaches directed to malignant liver lesions, bone metastases, neuroendocrine tumors, and castrate-resistant metastatic prostate cancer and discusses challenges and opportunities in this field.

CONCLUSION

TRT has been employed since the first radioiodine thyroid treatment almost 75 years ago. Progress in the understanding of the complex underlying biology of cancer and advances in radiochemistry science, multimodal imaging techniques including the concept of "see and treat" within the framework of theranostics, and universal traction with the notion of precision medicine have all contributed to a resurgence of TRT.

Targeted Radionuclide Therapy: Practical Applications and Future Prospects

In recent years, there has been a proliferation in the development of targeted radionuclide cancer therapy. It is now possible to use baseline clinical and imaging assessments to determine the most effective therapy and to tailor this therapy during the course of treatment based on radiation dosimetry and tumor response. Although this personalized approach to medicine has the advantage of maximizing therapeutic effect while limiting toxicity, it can be challenging to implement and expensive. Further, in order to use targeted radionuclide therapy effectively, there is a need for multidisciplinary awareness, education, and collaboration across the scientific, industrial, and medical communities. Even more important, there is a growing understanding that combining radiopharmaceuticals with conventional treatment such as chemotherapy and external beam radiotherapy may limit patient morbidity while improving survival. Developments in radiopharmaceuticals as biomarkers capable of predicting therapeutic response and targeting disease are playing a central role in medical research. Adoption of a practical approach to manufacturing and delivering radiopharmaceuticals, assessing patient eligibility, optimizing post-therapy follow-up, and addressing reimbursement issues will be essential for their success.

Translational radionanomedicine: a clinical perspective

Many nanomaterials were developed for the anticipated in vivo theranostic use exploiting their unique characteristics as a multifunctional platform. Nevertheless, only a few nanomaterials are under investigation for human use, most of which have not entered clinical trials yet. Radionanomedicine, a convergent discipline of radiotracer technology and use of nanomaterials in vivo, can facilitate clinical nanomedicine because of its advantages of radionuclide imaging and internal radiation therapy. In this review, we focuse on how radionanomedicine would impact profoundly on clinical translation of nanomaterial theranostics. Up-to-date advances and future challenges are critically reviewed regarding the issues of how to radiolabel and engineer radionanomaterials, in vivo behavior tracing of radionanomaterials and then the desired clinical radiation dosimetry. Radiolabeled extracellular vesicles were further discussed as endogenous nanomaterials radiolabeled for possible clinical use.

Beta emitters rhenium-188 and lutetium-177 are equally effective in radioimmunotherapy of HPV-positive experimental cervical cancer

Cervical cancer caused by the infection with the human papillomavirus (HPV) remains the fourth leading killer of women worldwide. Therefore, more efficacious treatments are needed. We are developing radioimmunotherapy (RIT) of HPV‐positive cervical cancers by targeting E6 and E7 viral oncoproteins expressed by the cancer cells with the radiolabeled monoclonal antibodies (mAbs). To investigate the influence of different radionuclides on the RIT efficacy—we performed RIT of experimental cervical cancer with Rhenium‐188 (¹⁸⁸Re) and Lutetium‐177 (¹⁷⁷Lu)‐labeled mAb C1P5 to E6. The biodistribution of ¹⁸⁸Re‐ and ¹⁷⁷Lu‐labeled C1P5 was performed in nude female mice bearing CasKi cervical cancer xenografts and the radiation dosimetry calculations for the tumors and organs were carried out. For RIT the mice were treated with 7.4 MBq of either ¹⁸⁸Re‐C1P5 or ¹⁷⁷Lu‐C1P5 or left untreated, and observed for their tumor size for 28 days. The levels of ¹⁸⁸Re‐ and ¹⁷⁷Lu‐C1P5 mAbs‐induced double‐strand breaks in CasKi tumors were compared on days 5 and 10 post treatment by staining with anti‐gamma H2AX antibody. The radiation doses to the heart and lungs were similar for both ¹⁷⁷Lu‐C1P5 and ¹⁸⁸Re‐C1P5. The dose to the liver was five times higher for ¹⁷⁷Lu‐C1P5. The doses to the tumor were 259 and 181 cGy for ¹⁷⁷Lu‐C1P5 and ¹⁸⁸Re‐C1P5, respectively. RIT with either ¹⁷⁷Lu‐C1P5 or ¹⁸⁸Re‐C1P5 was equally effective in inhibiting tumor growth when each was compared to the untreated controls (P = 0.001). On day 5 there was a pronounced staining for gamma H2AX foci in ¹⁷⁷Lu‐C1P5 group only and on day 10 it was observed in both ¹⁷⁷Lu‐C1P5 and ¹⁸⁸Re‐C1P5 groups. ¹⁸⁸Re‐ and ¹⁷⁷Lu‐labeled mAbs were equally effective in arresting the growth of CasKi cervical tumors. Thus, both of these radionuclides are candidates for the clinical trials of this approach in patients with advanced, recurrent or metastatic cervical cancer.

One-Year Postapproval Clinical Experience with Radium-223 Dichloride in Patients with Metastatic Castrate-Resistant Prostate Cancer

OBJECTIVES

We report our 1-year postapproval clinical experience with Radium-223 dichloride for treatment of castrate-resistant prostate cancer with bone metastases.

METHODS

The clinical courses of the first 25 patients treated were reviewed retrospectively. Incidence of hematologic, gastrointestinal, and other adverse events were identified, including those events that led to cessation or delay in treatment. Alterations in bone pain and serum alkaline phosphatase and prostate-specific antigen (PSA) levels were evaluated.

RESULTS

Six patients received all 6 scheduled doses of Radium-223 dichloride, 2 completed 5 doses, 6 received 4 doses, 2 completed 3 doses, 6 patients had 2 doses, and 3 patients received one dose, for a total of 91 doses administered. Nine patients discontinued treatment after receiving at least one dose due to progressive disease, 5 required blood transfusions, 5 developed gastrointestinal symptoms, 4 reported worsening bone pain, and 1 developed dermatitis. Downward trends in serum alkaline phosphatase and PSA were seen in 11 and 5 patients, respectively.

CONCLUSIONS

About one-quarter of cohort completed the entire six-dose treatment. Advancing soft tissue disease was the primary reason for cessation of therapy. The adverse events were mild and manageable. A decline in serum alkaline phosphatase was more common than a decline in PSA.

Advances in radiation biology: effect on nuclear medicine

Over the past 15 years and more, extensive research has been conducted on the responses of biological systems to radiation delivered at a low dose or low dose rate. This research has demonstrated that the molecular-, cellular-, and tissue-level responses are different following low doses than those observed after a single short-term high-dose radiation exposure. Following low-dose exposure, 3 unique responses were observed, these included bystander effects, adaptive protective responses, and genomic instability. Research on the mechanisms of action for each of these observations demonstrates that the molecular and cellular processes activated by low doses of radiation are often related to protective responses, whereas high-dose responses are often associated with extensive damage such as cell killing, tissue disruption, and inflammatory diseases. Thus, the mechanisms of action are unique for low-dose radiation exposure. When the dose is delivered at a low dose rate, the responses typically differ at all levels of biological organization. These data suggest that there must be a dose rate effectiveness factor that is greater than 1 and that the risk following low-dose rate exposure is likely less than that for single short-term exposures. All these observations indicate that using the linear no-threshold model for radiation protection purposes is conservative. Low-dose research therefore supports the current standards and practices. When a nuclear medical procedure is justified, it should be carried out with optimization (lowest radiation dose commensurate with diagnostic or therapeutic outcome).

A new approach for dose calculation in targeted radionuclide therapy (TRT) based on collapsed cone superposition: validation with (90)Y

To speed-up the absorbed dose (AD) computation while accounting for tissue heterogeneities, a Collapsed Cone (CC) superposition algorithm was developed and validated for (90)Y. The superposition was implemented with an Energy Deposition Kernel scaled with the radiological distance, along with CC acceleration. The validation relative to Monte Carlo simulations was performed on 6 phantoms involving soft tissue, lung and bone, a radioembolisation treatment and a simulated bone metastasis treatment. As a figure of merit, the relative AD difference (ΔAD) in low gradient regions (LGR), distance to agreement (DTA) in high gradient regions and the γ(1%,1 mm) criterion were used for the phantoms. Mean organ doses and γ(3%,3 mm) were used for the patient data. For the semi-infinite sources, ΔAD in LGR was below 1%. DTA was below 0.6 mm. All profiles verified the γ(1%,1 mm) criterion. For both clinical cases, mean doses differed by less than 1% for the considered organs and all profiles verified the γ(3%,3 mm). The calculation time was below 4 min on a single processor for CC superposition and 40 h on a 40 nodes cluster for MCNP (10(8) histories). Our results show that the CC superposition is a very promising alternative to MC for (90)Y dosimetry, while significantly reducing computation time.