Carnosic acid (CA) is a phenolic diterpene with high antioxidant activity that supports its radioprotective capacity. This study aims to determine whether the radiosensitizing effect of CA established in B16F10 melanoma cells also occurs in other melanin-producing cells. Cell survival analysis, apoptosis, intracellular glutathione levels, and cell cycle progression were evaluated by comparing radiosensitive cells (PNT2) with radioresistant melanin-producing cells (MELAN A, SK-MEL-1, and B16F10). In PNT2 cells, CA exhibited radioprotective capacity, with 100% cell survival after exposure to 20 Gy of X-rays (p < 0.001), decreasing apoptosis (p < 0.001) and increasing the GSH/GSSG ratio (p < 0.01), without significant modification in cell cycle progression. However, CA administration to irradiated cells failed to exert radioprotection in MELAN A and SK-MEL-1 cells, and even doubled cell death in B16F10 cells (p < 0.001). Specifically, CA did not alter apoptosis or prevent the decrease in GSH/GSSG ratio in MELAN A and SK-MEL-1 cells, while it intensified radiation-induced cell cycle disruptions in all melanin-producing cells. All of these led to a loss of radioprotective capacity in the melanin-producing cells (MELAN A and SK-MEL-1) and even induced a radiosensitizing effect in B16F10 cells. Understanding the mechanisms of action of substances such as CA could promote new applications that protect healthy cells and exclusively damage neoplastic cells when both are present within the same irradiated volume in cancer patients requiring radiotherapy.

There is an urgent need to find and develop radiation countermeasures for both planned and unplanned exposures. However, in recent years, radioprotective substances, such as rosmarinic acid (RA), have been described, presenting contradictory and even paradoxical results. In this paper, we evaluated the genoprotective capacity of 29 substances against chromosomal damage induced by gamma radiation in a comparative study using the same technique, i.e., cytokinesis-block micronucleus (CBMN) assay to evaluate their genoprotective capacities, at the same concentrations and administered before and after exposure to 2 Gy of gamma radiation. We then related the observed effects with their chemical characteristics to appreciate the different mechanisms of action that could explain some apparent contradictions that may emerge. In our study, before exposure to ionizing radiation, RA produced the greatest reduction in the frequency of radiation-induced micronuclei (p < 0.001), presenting the highest magnitude of protection (58%) and a dose reduction factor of 7.1 (p < 0.001); however, it loses this genoprotective capacity when administered after exposure to radiation. These results could be attributed to the different radical-scavenging and antilipoperoxidative activities of each substance tested. Antilipoperoxidant activity was found to be the most important factor in the reduction in radiation-induced chromosomal damage; thus, lipo-antioxidant substances emerged as the most effective in protecting genetic material against oxidative damage.

Carnosic acid (CA) is a phenolic diterpene with high antioxidant activity that supports its radioprotective capacity. This study aims to determine whether the radiosensitizing effect of CA established in B16F10 melanoma cells also occur in other mela-nin-producing cells. Cell survival analysis, apoptosis, intracellular glutathione levels, and cell cycle progression were evaluated by comparing radiosensitive cells (PNT2) with radioresistant melanin-producing cells (MELAN A, SK-MEL-1 and B16F10). In PNT2 cells, CA exhibited radioprotective capacity, with 100% cell survival after expo-sure to 20 Gy of X-rays (p < 0.001), decreasing apoptosis (p < 0.001) and increasing the GSH/GSSH ratio (p < 0.01), without significant modification in cell cycle progression. However, the administration of CA to irradiated cells failed to show radioprotective capacity in MELAN A and SK-MEL-1 cells, it even doubled cell death in B16F10 cells (p < 0.001); did not modify cell apoptosis, nor prevent a decrease in the GSH/GSSG ratio and intensify the modifications in the cell cycle induced by ionizing radiation. All of these led to a loss of radioprotective capacity in the melanin producing cells (MELAN A and SK-MEL-1) and even induced a radiosensitizing effect in B16F10 cells. Understanding the mechanisms of action of substances such as CA could promote new applications that protect healthy cells and exclusively damage neoplastic cells when both are present within the same irradiated volume in cancer patients requiring radiotherapy.

Published cytokinesis-block micronucleus (CBMN) frequencies, normalised to 500 binucleated cells (MN/500 BC), were used to illustrate how the choice of statistical parametrisation affects background-subtracted radioprotection metrics. An illustrative Jeffreys-rule transformation was applied to published normalised frequencies from eight representative experimental groups, drawn from two previously published human-lymphocyte CBMN datasets generated by a single laboratory, within the broader retrospective context of four source studies (2000–2026): an irradiated control, four pre-irradiation radioprotective treatments, one post-irradiation treatment, one radiosensitiser, and an iodine-131 biodosimetry cohort. It was not used as a statistically valid Bayesian inference based on the original experimental counts. Because the original integer micronucleus counts and the exact numbers of binucleated cells scored per condition could not be recovered, the analysis is presented as a descriptive methodological comparison. On the normalised scale, the transformation increased the net MN value when observed frequencies were close to background, with a maximum correction of 2.38 MN/500 BC, and approached the analytically expected asymptotic offset of 0.5 MN/500 BC under the illustrative working parametrisation, as the observed values substantially exceeded the background. Consequently, within the selected groups analysed, the apparent magnitude of protection calculated from the transformed values was lower than the classical estimates for the selected pre-irradiation treatments, showing the largest classical MP values—for example, 79.5% versus 93.8% for rosmarinic acid. The ordering of the selected radioprotective-treatment groups by MP was unchanged. These results illustrate the sensitivity of background-subtracted MN metrics to statistical parametrisation under low-signal conditions. They must not be interpreted as valid experimental uncertainty estimates, as dose estimates, or as clinical decision criteria.

Jeffreys‑rule Bayesian inference for the Poisson mean was retrospectively applied to cytokinesis‑block micronucleus (CBMN) data from four studies conducted in a single laboratory (2001–2026), covering ionising‑radiation exposures in human lymphocytes and an ¹³¹I biodosimetry study (29 experimental groups). The CBMN assay is widely used for cytogenetic biodosimetry and for evaluating radioprotective agents, but classical analysis of net micronucleus (MN) yield becomes unreliable when effective radioprotectors reduce MN frequencies towards background, leading to near‑zero or negative net counts with poorly characterised uncertainty. In our reanalysis, the Bayesian correction systematically modified magnitude-of-protection and dose-reduction estimates in low-count, high‑protection conditions in comparison with the classical approach. Using Bayesian net counts to recalculate standard radioprotective metrics revealed that, while the overall ranking of radioprotective potency was preserved, Bayesian magnitudes of protection and dose reduction factors were consistently lower than their classical counterparts for the most effective radioprotectors administered before irradiation, providing more conservative efficacy estimates with quantified uncertainty. In the ¹³¹I biodosimetry cohort and in patients undergoing diagnostic nuclear medicine procedures, post‑exposure MN yields remained within the spontaneous background variability of the assay, and the Bayesian framework is fully compatible with the practical absence of detectable chromosomal damage at the doses used in routine clinical practice. Taken together, these results demonstrate that Jeffreys‑rule Bayesian inference provides a statistically coherent and practically feasible framework for CBMN analysis in the low‑count regime, stabilising net MN estimates, avoiding artefactual negative values and supplying confidence intervals for radioprotective efficacy metrics. As a concrete methodological recommendation, Bayesian analysis should be adopted as the default approach whenever post‑exposure MN counts are within approximately a factor of two of background, with classical methods remaining adequate at higher count levels.