Absorbed dose simulation of meta-211At-astato-benzylguanidine using pharmacokinetics of 131I-MIBG and a novel dose conversion method, RAP

Evaluation of pharmacokinetics, safety, and efficacy of [211At] meta-astatobenzylguanidine ([211At] MABG) in patients with pheochromocytoma or paraganglioma (PPGL): A study protocol

BACKGROUND

Pheochromocytoma, or paraganglioma (PPGL), is a tumor that arises from catecholamine-producing chromaffin cells of the adrenal medulla or paraganglion. Systemic therapy, such as the combination of cyclophosphamide, vincristine, and dacarbazine or therapeutic radiopharmaceuticals such as [131I] meta-iodobenzylguanidine (MIBG), may be administered in cases of locally advanced tumors or distant metastases. However, the current therapies are limited in terms of efficacy and implementation. [211At] meta-astatobenzylguanidine (MABG) is an alpha-emitting radionuclide-labeled ligand that has demonstrated remarkable tumor-reducing effects in preclinical studies, and is expected to have a high therapeutic effect on pheochromocytoma cells.

METHODS

We are currently conducting an investigator-initiated first-in-human clinical trial to evaluate the pharmacokinetics, safety, and efficacy of [211At] MABG. Patients with locally unresectable or metastatic PPGL refractory to standard therapy and scintigraphically positive [123I] MIBG aggregation are being recruited, and a 3 + 3 dose escalation design was adopted. The initial dose of [211At] MABG is 0.65 MBq/kg, with a dose escalation in a 1:2:4 ratio in each cohort. Dose-limiting toxicity is observed for 6 weeks after a single bolus dose of [211At] MABG, and the patients are observed for 3 months to explore safety and efficacy profiles. The primary endpoint is dose-limiting toxicity to determine both maximum tolerated and recommended doses. The secondary endpoints include radiopharmacokinetics, urinary radioactive excretion rate, urinary catecholamine response rate, objective response rate, progression free survival, [123I] MIBG scintigraphy on reducing tumor accumulation, and quality of life.

TRIALS REGISTRATION

jRCT2021220012 registered on 17 June 2022.

Comparative evaluation of radionuclide therapy using 90Y and 177Lu

OBJECTIVE

Both ⁹⁰Y and ¹⁷⁷Lu are attractive β-emitters for radionuclide therapy and have been used in clinical practice. Nevertheless, comparative evaluation between ⁹⁰Y- and ¹⁷⁷Lu-labeled molecules has not been fully conducted. Thus, in this study, the features of ⁹⁰Y and ¹⁷⁷Lu for radionuclide therapy were assessed in tumor-bearing mice.

METHODS

Two tumor cell lines with different growth rates were used. Biodistribution studies of ¹⁷⁷Lu-labeled antibodies (¹⁷⁷Lu-Abs) were conducted in each tumor-bearing mouse model. Subsequently, the therapeutic effect of ⁹⁰Y- and ¹⁷⁷Lu-Ab were assessed in tumor-bearing mice. The absorbed radiation dose for the tumor was estimated using the Monte Carlo simulation.

RESULTS

¹⁷⁷Lu-Abs demonstrated high tumor accumulation in both tumor-xerograph. In the fast-growing tumor model, ⁹⁰Y-Ab showed a better therapeutic effect than ¹⁷⁷Lu-Ab, reflecting a higher absorbed radiation dose of ⁹⁰Y-Ab than that of ¹⁷⁷Lu-Ab. In the slow-growing tumor model, both ⁹⁰Y- and ¹⁷⁷Lu-Ab showed an excellent therapeutic effect; however, ¹⁷⁷Lu-Ab had a longer efficacy period than ⁹⁰Y-Ab, which could be attributed to the longer half-life and better dose uniformity of ¹⁷⁷Lu than those of ⁹⁰Y.

CONCLUSIONS

To accomplish a maximum therapeutic effect, selecting ⁹⁰Y or ¹⁷⁷Lu, to depend on the growth rate of individual cancer, would be helpful.

Nonclinical study and applicability of the absorbed dose conversion method with a single biodistribution measurement for targeted alpha-nuclide therapy

Background

We recently reported a new absorbed dose conversion method, RAP (RAtio of Pharmacokinetics), for ²¹¹At-meta-astatobenzylguanidine (²¹¹At-MABG) using a single biodistribution measurement, the percent injected dose/g. However, there were some mathematical ambiguities in determining the optimal timing of a single measurement of the percent injected dose/g. Thus, we aimed to mathematically reconstruct the RAP method and to examine the optimal timing of a single measurement.

Methods

We derived a new formalism of the RAP dose conversion method at time t. In addition, we acquired a formula to determine the optimal timing of a single measurement of the percent injected dose/g, assuming the one-compartment model for biological clearance.

Results

We investigated the new formalism’s performance using a representative RAP coefficient with radioactive decay weighting. Dose conversions by representative RAP coefficients predicted the true [²¹¹At]MABG absorbed doses with an error of 10% or less. The inverses of the representative RAP coefficients plotted at 4 h post-injection, which was the optimal timing reported in the previous work, were very close to the new inverses of the RAP coefficients 4 h post-injection. Next, the behavior of the optimal timing was analyzed by radiolabeled compounds with physical half-lives of 7.2 h and 10 d on various biological clearance half-lives. Behavior maps of optimal timing showed a tendency to converge to a constant value as the biological clearance half-life of a target increased. The areas of optimal timing for both compounds within a 5% or 10% prediction error were distributed around the optimal timing when the biological clearance half-life of a target was equal to that of the reference. Finally, an example of RAP dose conversion was demonstrated for [²¹¹At]MABG.

Conclusions

The RAP dose conversion method renovated by the new formalism was able to estimate the [²¹¹At]MABG absorbed dose using a similar pharmacokinetics, such as [¹³¹I]MIBG. The present formalism revealed optimizing imaging time points on absorbed dose conversion between two radiopharmaceuticals. Further analysis and clinical data will be needed to elucidate the validity of a behavior map of the optimal timing of a single measurement for targeted alpha-nuclide therapy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40658-021-00425-z.