Real-time, volumetric imaging of radiation dose delivery deep into the liver during cancer treatment

Ionizing radiation acoustic imaging (iRAI) allows online monitoring of radiation's interactions with tissues during radiation therapy, providing real-time, adaptive feedback for cancer treatments. We describe an iRAI volumetric imaging system that enables mapping of the three-dimensional (3D) radiation dose distribution in a complex clinical radiotherapy treatment. The method relies on a two-dimensional matrix array transducer and a matching multi-channel preamplifier board. The feasibility of imaging temporal 3D dose accumulation was first validated in a tissue-mimicking phantom. Next, semiquantitative iRAI relative dose measurements were verified in vivo in a rabbit model. Finally, real-time visualization of the 3D radiation dose delivered to a patient with liver metastases was accomplished with a clinical linear accelerator. These studies demonstrate the potential of iRAI to monitor and quantify the 3D radiation dose deposition during treatment, potentially improving radiotherapy treatment efficacy using real-time adaptive treatment.

Preclinical evaluation of [225Ac]Ac-DOTA-TATE for treatment of lung neuroendocrine neoplasms

PURPOSE

There is significant interest in the development of targeted alpha-particle therapies (TATs) for treatment of solid tumors. The metal chelator-peptide conjugate, DOTA-TATE, loaded with the β-particle emitting radionuclide ¹⁷⁷Lu ([¹⁷⁷Lu]Lu-DOTA-TATE) is now standard care for neuroendocrine tumors that express the somatostatin receptor 2 (SSTR2) target. A recent clinical study demonstrated efficacy of the corresponding [²²⁵Ac]Ac-DOTA-TATE in patients that were refractory to [¹⁷⁷Lu]Lu-DOTA-TATE. Herein, we report the radiosynthesis, toxicity, biodistribution (BD), radiation dosimetry (RD), and efficacy of [²²⁵Ac]Ac-DOTA-TATE in small animal models of lung neuroendocrine neoplasms (NENs).

METHODS

[²²⁵Ac]Ac-DOTA-TATE was synthesized and characterized for radiochemical yield, purity and stability. Non-tumor-bearing BALB/c mice were tested for toxicity and BD. Efficacy was determined by single intravenous injection of [²²⁵Ac]Ac-DOTA-TATE into SCID mice-bearing human SSTR2 positive H727 and H69 lung NENs. RD was calculated using the BD data.

RESULTS

[²²⁵Ac]Ac-DOTA-TATE was synthesized with 98% yield, 99.8% purity, and displayed 97% stability after 2 days incubation in human serum at 37 °C. All animals in the toxicity study appeared healthy 5 months post injection with no indications of toxicity, except that animals that received ≥111 kBq of [²²⁵Ac]Ac-DOTA-TATE had chronic progressive nephropathy. BD studies revealed that the primary route of elimination is by the renal route. RD calculations determined pharmacokinetics parameters and absorbed α-emission dosages from ²²⁵Ac and its daughters. For both tumor models, a significant tumor growth delay and time to experimental endpoint were observed following a single administration of [²²⁵Ac]Ac-DOTA-TATE relative to controls.

CONCLUSIONS

These results suggest significant potential for the clinical translation of [²²⁵Ac]Ac-DOTA-TATE for lung NENs.

Lipophilicity Determines Routes of Uptake and Clearance, and Toxicity of an Alpha-Particle-Emitting Peptide Receptor Radiotherapy

Lipophilicity is explored in the biodistribution (BD), pharmacokinetics (PK), radiation dosimetry (RD), and toxicity of an internally administered targeted alpha-particle therapy (TAT) under development for the treatment of metastatic melanoma. The TAT conjugate is comprised of the chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate), conjugated to melanocortin receptor 1 specific peptidic ligand (MC1RL) using a linker moiety and chelation of the ²²⁵Ac radiometal. A set of conjugates were prepared with a range of lipophilicities (log D _7.4 values) by varying the chemical properties of the linker. Reported are the observations that higher log D _7.4 values are associated with decreased kidney uptake, decreased absorbed radiation dose, and decreased kidney toxicity of the TAT, and the inverse is observed for lower log D _7.4 values. Animals administered TATs with lower lipophilicities exhibited acute nephropathy and death, whereas animals administered the highest activity TATs with higher lipophilicities lived for the duration of the 7 month study and exhibited chronic progressive nephropathy. Changes in TAT lipophilicity were not associated with changes in liver uptake, dose, or toxicity. Significant observations include that lipophilicity correlates with kidney BD, the kidney-to-liver BD ratio, and weight loss and that blood urea nitrogen (BUN) levels correlated with kidney uptake. Furthermore, BUN was identified as having higher sensitivity and specificity of detection of kidney pathology, and the liver enzyme alkaline phosphatase (ALKP) had high sensitivity and specificity for detection of liver damage associated with the TAT. These findings suggest that tuning radiopharmaceutical lipophilicity can effectively modulate the level of kidney uptake to reduce morbidity and improve both safety and efficacy.

Biodistribution and Multicompartment Pharmacokinetic Analysis of a Targeted α Particle Therapy

Targeted α particle therapy (TAT) is ideal for treating disease while minimizing damage to surrounding nontargeted tissues due to short path length and high linear energy transfer (LET). We developed a TAT for metastatic uveal melanoma, targeting the melanocortin-1 receptor (MC1R), which is expressed in 94% of uveal melanomas. Two versions of the therapy are being investigated: ²²⁵Ac-DOTA-Ahx-MC1RL (²²⁵Ac-Ahx) and ²²⁵Ac-DOTA-di-d-Glu-MC1RL (²²⁵Ac-di-d-Glu). The biodistribution (BD) from each was studied and a multicompartment pharmacokinetic (PK) model was developed to describe drug distribution rates. Two groups of 16 severe combined immunodeficient (SCID) mice bearing high MC1R expressing tumors were intravenously injected with ²²⁵Ac-Ahx or ²²⁵Ac-di-d-Glu. After injection, four groups (n = 4) were euthanized at 24, 96, 144, and 288 h time points for each cohort. Tumors and 13 other organs were harvested at each time point. Isomeric γ spectra were measured in tissue samples using a scintillation γ detector and converted to α activity using factors for γ ray abundance per α decay. Time activity curves were calculated for each organ. A five-compartment PK model was built with the following compartments: blood, tumor, normal tissue, kidney, and liver. This model is characterized by a system of five ordinary differential equations using mass action kinetics, which describe uptake, intercompartmental transitions, and clearance rates. The ordinary differential equations were simultaneously solved and fit to experimental data using a genetic algorithm for optimization. The BD data show that both compounds have minimal distribution to organs at risk other than the kidney and liver. The PK parameter estimates had less than 5% error. From these data, ²²⁵Ac-Ahx showed larger and faster uptake in the liver. Both compounds had comparable uptake and clearance rates for other compartments. The BD and PK behavior for two targeted radiopharmaceuticals were investigated. The PK model fit the experimental data and provided insight into the kinetics of the compounds systematically.

Development of Targeted Alpha Particle Therapy for Solid Tumors

Targeted alpha-particle therapy (TAT) aims to selectively deliver radionuclides emitting α-particles (cytotoxic payload) to tumors by chelation to monoclonal antibodies, peptides or small molecules that recognize tumor-associated antigens or cell-surface receptors. Because of the high linear energy transfer (LET) and short range of alpha (α) particles in tissue, cancer cells can be significantly damaged while causing minimal toxicity to surrounding healthy cells. Recent clinical studies have demonstrated the remarkable efficacy of TAT in the treatment of metastatic, castration-resistant prostate cancer. In this comprehensive review, we discuss the current consensus regarding the properties of the α-particle-emitting radionuclides that are potentially relevant for use in the clinic; the TAT-mediated mechanisms responsible for cell death; the different classes of targeting moieties and radiometal chelators available for TAT development; current approaches to calculating radiation dosimetry for TATs; and lead optimization via medicinal chemistry to improve the TAT radiopharmaceutical properties. We have also summarized the use of TATs in pre-clinical and clinical studies to date.

A Monte Carlo Method for Determining the Response Relationship between Two Commonly Used Detectors to Indirectly Measure Alpha Particle Radiation Activity

Using targeted ligands to deliver alpha-emitting radionuclides directly to tumor cells has become a promising therapeutic strategy. To calculate the radiation dose to patients, activities of parent and daughter radionuclides must be measured. Scintillation detectors can be used to quantify these activities; however, activities found in pre-clinical and clinical studies can exceed their optimal performance range. Therefore, a method of correcting scintillation detector measurements at higher activities was developed using Monte Carlo modeling. Because there are currently no National Institute of Standards and Technology traceable Actinium-225 (²²⁵Ac) standards available, a well-type ionization chamber was used to measure 70.3 ± 7.0, 144.3 ± 14.4, 222.0 ± 22.2, 299.7 ± 30.0, 370.0 ± 37.0, and 447.7 ± 44.7 kBq samples of ²²⁵Ac obtained from Oak Ridge National Lab. Samples were then placed in a well-type NaI(Tl) scintillation detector and spectra were obtained. Alpha particle activity for each species was calculated using gamma abundance per alpha decay. MCNP6 Monte Carlo software was used to simulate the 4π-geometry of the NaI(Tl) detector. Using the ionization chamber reading as activity input to the Monte Carlo model, spectra were obtained and compared to NaI(Tl) spectra. Successive simulations of different activities were run until a spectrum minimizing the mean percent difference between the two was identified. This was repeated for each sample activity. Ionization chamber calibration measurements showed increase in error from 3% to 10% as activities decreased, resulting from decreasing detection efficiency. Measurements of ²²⁵Ac using both detector types agreed within 7% of Oak Ridge stated activities. Simulated Monte Carlo spectra of ²²⁵Ac were successfully generated. Activities obtained from these spectra differed with ionization chamber readings up to 156% at 147.7 kBq. Simulated spectra were then adjusted to correct NaI(Tl) measurements to be within 1%. These were compared to ionization chamber readings and a response relationship was determined between the two instruments. Measurements of ²²⁵Ac and daughter activity were conducted using a NaI(Tl) scintillation detector calibrated for energy and efficiency and an ionization chamber calibrated for efficiency using a surrogate calibration reference. Corrections provided by Monte Carlo modeling improve the accuracy of activity quantification for alpha-particle emitting radiopharmaceuticals in pre-clinical and clinical studies.

Melanocortin 1 Receptor-Targeted α-Particle Therapy for Metastatic Uveal Melanoma

New effective therapies are greatly needed for metastatic uveal melanoma, which has a very poor prognosis with a median survival of less than 1 y. The melanocortin 1 receptor (MC1R) is expressed in 94% of uveal melanoma metastases, and a MC1R-specific ligand (MC1RL) with high affinity and selectivity for MC1R was previously developed. Methods: The ²²⁵Ac-DOTA-MC1RL conjugate was synthesized in high radiochemical yield and purity and was tested in vitro for biostability and for MC1R-specific cytotoxicity in uveal melanoma cells, and the lanthanum-DOTA-MC1RL analog was tested for binding affinity. Non-tumor-bearing BALB/c mice were tested for maximum tolerated dose and biodistribution. Severe combined immunodeficient mice bearing uveal melanoma tumors or engineered MC1R-positive and -negative tumors were studied for biodistribution and efficacy. Radiation dosimetry was calculated using mouse biodistribution data and blood clearance kinetics from Sprague-Dawley rat data. Results: High biostability, MC1R-specific cytotoxicity, and high binding affinity were observed. Limiting toxicities were not observed at even the highest administered activities. Pharmacokinetics and biodistribution studies revealed rapid blood clearance (<15 min), renal and hepatobillary excretion, MC1R-specific tumor uptake, and minimal retention in other normal tissues. Radiation dosimetry calculations determined pharmacokinetics parameters and absorbed α-emission dosages from ²²⁵Ac and its daughters. Efficacy studies demonstrated significantly prolonged survival and decreased metastasis burden after a single administration of ²²⁵Ac-DOTA-MC1RL in treated mice relative to controls. Conclusion: These results suggest significant potential for the clinical translation of ²²⁵Ac-DOTA-MC1RL as a novel therapy for metastatic uveal melanoma.

Abstract 5198: Targeted alpha particle therapy for uveal melanoma

New effective therapies are greatly needed for uveal melanoma, the most common ocular malignancy, which has a very poor prognosis with a median survival of less than one year when metastatic disease develops. The melanocortin 1 receptor (MC1R) is expressed in 94% of uveal melanomas but is not expressed in normal tissues of concern for toxicity. Previously, we developed an MC1R specific ligand (MC1RL) with high affinity and selectivity for MC1R. We used the MC1RL as a targeting scaffold for development of a radiopharmaceutical by conjugation of 225Ac chelate to form 225Ac -MC1RL-DOTA. 225Ac is a therapeutic alpha-emitting radionuclide. 225Ac -MC1RL-DOTA was synthesized with high radiochemical yield and purity (&gt;95 and &gt;99 % respectively) with an excellent in vitro stability, i.e. 90% after 10 days in human serum at 37ºC. An MTT cytotoxicity study demonstrated MC1R specific antiproliferative effect in cutaneous, ocular and uveal melanoma cell lines. A maximum tolerated dose (MTD) study was completed, where a range of 0-148 kBq (0-4 µCi) of 225Ac -MC1RL-DOTA was administered to groups of BALB/c mice. The radiopharmaceutical was well tolerated at even the highest doses and animals did not reach any clinical endpoints, such as weight loss, loss of kidney function or abnormal pathology. Biodistribution studies on MC1R expressing tumor bearing mice revealed tumor selectivity and a combination of renal and hepatic clearance with minimal retention in other normal tissues. A blood PK study in rats showed rapid clearance of the agent from the blood in &lt;60 min. To determine radiodosimetry, gamma spectra were acquired for tumors, blood and other tissues and organs over a 21-day time period and activity curves generated for each tissue for 225Ac and other gamma-emitting daughter products in the decay chain. An exponential line-fitting of these curves allowed the estimation of clearance kinetics, tissue biological half-life and total absorbed dose (Gy) for each tissue. Doses were highest in MC1R positive tumors and clearance organs (liver, kidney and spleen) and low in all other tissues. In vivo efficacy studies were performed in human A375/MC1R cutaneous and MEL20 uveal melanoma tumors in SCID mice, or syngeneic mouse B16 melanoma tumors in C57BL/6 mice (n=10/group) injected with either 225Ac -MC1RL-DOTA, 225Ac-scrambled-MC1RL-DOTA, La-MC1RL-DOTA (non-radioactive control agent) or saline. In every case a tumor growth delay was observed in treated mice relative to controls, and a percentage of mice bearing the human tumors had complete remission. In conclusion, we evaluated the in vitro and in vivo properties of 225Ac-MC1RL-DOTA in this study. This agent demonstrated a significant MC1R-specific in vivo therapeutic efficacy in a mouse model of uveal melanoma with low systemic toxicity.

Citation Format: Narges K. Tafreshi, Nella C. Delva, Christopher J. Tichacek, Michael L. Doligalski, Darpan N. Pandya, Nikunj B. Bhatt, HyunJoo Kil, Mikalai M. Budzevich, Epifanio Ruiz, Thaddeus J. Wadas, Mark L. McLaughlin, David L. Morse. Targeted alpha particle therapy for uveal melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5198. doi:10.1158/1538-7445.AM2017-5198