Localized in vivo prodrug activation using radionuclides

Radionuclides used for imaging and therapy can show high molecular specificity in the body with appropriate targeting ligands. We hypothesized that local energy delivered by molecularly targeted radionuclides could chemically activate prodrugs at disease sites while avoiding activation in off-target sites of toxicity. As proof-of-principle, we tested whether this strategy of " RA dionuclide i nduced D rug E ngagement for R elease" ( RAiDER ) could locally deliver combined radiation and chemotherapy to maximize tumor cytotoxicity while minimizing exposure to activated chemotherapy in off-target sites.

Methods

We screened the ability of radionuclides to chemically activate a model radiation-activated prodrug consisting of the microtubule destabilizing monomethyl auristatin E caged by a radiation-responsive phenyl azide ("caged-MMAE") and interpreted experimental results using the radiobiology computational simulation suite TOPAS-nBio. RAiDER was evaluated in syngeneic mouse models of cancer using fibroblast activation protein inhibitor (FAPI) agents 99m Tc-FAPI-34 and 177 Lu-FAPI-04, the prostate-specific membrane antigen (PSMA) agent 177 Lu-PSMA-617, combined with caged-MMAE or caged-exatecan. Biodistribution in mice, combined with clinical dosimetry, estimated the relationship between radiopharmaceutical uptake in patients and anticipated concentrations of activated prodrug using RAiDER.

Results

RAiDER efficiency varied by 250-fold across radionuclides ( 99m Tc> 177 Lu> 64 Cu> 68 Ga> 223 Ra> 18 F), yielding up to 1.22µM prodrug activation per Gy of exposure from 99m Tc. Computational simulations implicated low-energy electron-mediated free radical formation as driving prodrug activation. Clinically relevant radionuclide concentrations chemically activated caged-MMAE restored its ability to destabilize microtubules and increased its cytotoxicity by up to 600-fold compared to non-irradiated prodrug. Mice treated with 99m Tc-FAPI-34 and caged-MMAE accumulated up to 3000× greater concentrations of activated MMAE in tumors compared to other tissues. RAiDER with 99m Tc-FAPI-34 or 177 Lu-FAPI-04 delayed tumor growth, while monotherapies did not ( P <0.03). Clinically-guided dosimetry suggests sufficient radiation doses can be delivered to activate therapeutically meaningful levels of prodrug.

Conclusion

This proof-of-concept study shows that RAiDER is compatible with multiple radionuclides commonly used in nuclear medicine and has the potential to improve the efficacy of radiopharmaceutical therapies to treat cancer safely. RAiDER thus shows promise as an effective strategy to treat disseminated malignancies and broadens the capability of radiopharmaceuticals to trigger diverse biological and therapeutic responses.

Abstract Figure

Scavenging of “dry” electrons prior to hydration by azide ions: effect on the formation of H2 in the radiolysis of water by 60Co γ-rays and tritium β-electrons

In this study, we use Monte Carlo track chemistry simulations to show that “dry” secondary electrons, precursors of the “hydrated” electron (e−aq), can be scavenged on the sub-picosecond time scale prior to hydration, by a high concentration (>0.1–1 M) of azide ions (N3−) in water irradiated with 60Co γ-rays and tritium β-electrons at 25 °C. This is a striking result, as N3− is known to react very slowly with e−aq. These processes tend to significantly reduce the yields of H2 as observed experimentally. For both energetic Compton electrons (“linear energy transfer”, LET ∼ 0.3 keV/µm), which are generated by the cobalt-60 γ-rays, and 3H β-electrons (LET ∼ 6 keV/µm), our H2 yield results confirm previous Monte Carlo simulations, which indicated the necessity of including the capture of the precursors to e−aq. Interestingly, our calculations show no significant changes in the scavenging of “dry” electrons at high azide concentrations in passing from γ-radiolysis to tritium β-radiolysis (i.e., with LET). This led us to the conclusion that the higher H2 yield observed experimentally for 3H β-electrons compared with 60Co γ-rays is mainly explained by the difference in the radiation track structures during the chemical stage (>1 ps). The higher LET of tritium β-electrons leads to more molecular products (H2 in this case) in tritium radiolysis than in γ-radiolysis. Finally, a value of ∼0.5 nm was derived for the reaction distance between N3− and the “dry” electron from the H2 yields observed in 60Co γ-radiolysis at high N3− concentrations.

Generation of ultrafast, transient, highly acidic pH spikes in the radiolysis of water at very high dose rates: relevance for FLASH radiotherapy

Monte Carlo multi-track chemistry simulations were carried out to study the effects of high dose rates on the transient yields of hydronium ions (H3O+) formed during low linear energy transfer (LET) radiolysis of both pure, deaerated and aerated liquid water at 25 °C, in the interval ∼1 ps – 10 μs. Our simulation model consisted of randomly irradiating water with N interactive tracks of 300-MeV incident protons (LET ∼ 0.3 keV/µm), which simultaneously impact perpendicularly on the water within a circular surface. The effect of the dose rate was studied by varying N. Our calculations showed that the radiolytic formation of H3O+ causes the entire irradiated volume to temporarily become very acidic. The magnitude and duration of this abrupt “acid-spike” response depend on the value of N. It is most intense at times less than ∼10–100 ns, equal to ∼3.4 and 2.8 for N = 500 and 2000 (i.e., for dose rates of ∼1.9 × 109 and 8.7 × 109 Gy/s, respectively). At longer times, the pH gradually increases for all N values and eventually returns to the neutral value of seven, which corresponds to the non-radiolytic, pre-irradiation concentration of H3O+. It is worth noting that these early acidic pH responses are very little dependent on the presence or absence of oxygen. Finally, given the importance of pH for many cellular functions, this study suggests that these acidic pH spikes may contribute to the normal tissue-sparing effect of FLASH radiotherapy.

Transient hypoxia in water irradiated by swift carbon ions at ultra-high dose rates: implication for FLASH carbon-ion therapy

Large doses of ionizing radiation delivered to tumors at ultra-high dose rates (i.e., in a few milliseconds) paradoxically spare the surrounding healthy tissue while preserving anti-tumor activity (compared with conventional radiotherapy delivered at much lower dose rates). This new modality is known as “FLASH radiotherapy” (FLASH-RT). Although the molecular mechanisms underlying FLASH-RT are not yet fully understood, it has been suggested that radiation delivered at high dose rates spares normal tissue via oxygen depletion followed by subsequent radioresistance of the irradiated tissue. To date, FLASH-RT has been studied using electrons, photons, and protons in various basic biological experiments, pre-clinical studies, and recently in a human patient. However, the efficacy of heavy ions, such as energetic carbon ions, under FLASH-RT conditions remains unclear. Given that living cells and tissues consist mainly of water, we set out to study, from a pure radiation chemistry perspective, the effects of ultra-high dose rates on the transient yields and concentrations of radiolytic species formed in water irradiated by 300-MeV per nucleon carbon ions (LET ∼ 11.6 keV/µm). This mimics irradiation in the “plateau” region of the depth–dose distribution of ions, i.e., in the “normal” tissue region in which the LET is rather low. We used Monte Carlo simulations of multiple, simultaneously interacting radiation tracks together with an “instantaneous pulse” irradiation model. Our calculations show a pronounced oxygen depletion around 0.2 μs, strongly suggesting, as with electrons, photons, and protons, that irradiation with energetic carbon ions at ultra-high dose rates is suitable for FLASH-RT.

Yield of the Fricke dosimeter irradiated with the recoil α and Li ions of the 10B(n,α)7Li nuclear reaction: effects of multiple ionization and temperature

Monte Carlo track chemistry simulations were used to investigate the effects of multiple ionization (MI) of water on the yields (G values) of the ferrous sulfate (Fricke) dosimeter, which was irradiated with low-energy α and lithium ion recoils from the 10B(n,α)7Li nuclear reaction as a function of temperature from 25 to 350 °C. Calculations were performed individually for 1.47 MeV α-particles and 0.84 MeV lithium nuclei with dose-average linear energy transfer (LET) values of ∼196 and 225 keV/µm at 25 °C, respectively. The total yields were obtained by summing the G values for each recoil α and Li ion weighted with its fraction of the total energy absorbed. At room temperature, our G(Fe3+) values calculated under aerated and deaerated conditions only agreed well with the experimental results, provided the MI of water was incorporated in the simulations. This strongly supports the importance of the role of MI of water in the high-LET radiolysis of water. We also simulated the effects of MI of water on G-values for the primary species of the radiolysis of deaerated 0.4 M H2SO4 aqueous solutions by 10B(n,α)7Li recoils. As with the Fricke dosimeter, the best agreement between experiment and simulation was found at 25 °C when the MI of water was included in the simulations. It was also shown that G(Fe3+) decreases slightly as a function of temperature over the range of 25–350 °C. However, at elevated temperatures, no experimental data were available with which to compare our results.

Ultra-High Dose-Rate, Pulsed (FLASH) Radiotherapy with Carbon Ions: Generation of Early, Transient, Highly Oxygenated Conditions in the Tumor Environment

It is well known that molecular oxygen is a product of the radiolysis of water with high-linear energy transfer (LET) radiation, which is distinct from low-LET radiation wherein O2 radiolytic yield is negligible. Since O2 is a powerful radiosensitizer, this fact is of practical relevance in cancer therapy with energetic heavy ions, such as carbon ions. It has recently been discovered that large doses of ionizing radiation delivered to tumors at very high dose rates (i.e., in a few milliseconds) have remarkable benefits in sparing healthy tissue while preserving anti-tumor activity compared to radiotherapy delivered at conventional, lower dose rates. This new method is called "FLASH radiotherapy" and has been tested using low-LET radiation (i.e., electrons and photons) in various pre-clinical studies and recently in a human patient. Although the exact mechanism(s) underlying FLASH are still unclear, it has been suggested that radiation delivered at high dose rates spares normal tissue via oxygen depletion. In addition, heavy-ion radiation achieves tumor control with reduced normal tissue toxicity due to its favorable physical depth-dose profile and increased radiobiological effectiveness in the Bragg peak region. To date, however, biological research with energetic heavy ions delivered at ultra-high dose rates has not been performed and it is not known whether heavy ions are suitable for FLASH radiotherapy. Here we present the additive or even synergistic advantages of integrating the FLASH dose rates into carbon-ion therapy. These benefits result from the ability of heavy ions at high LET to generate an oxygenated microenvironment around their track due to the occurrence of multiple (mainly double) ionization of water. This oxygen is abundant immediately in the tumor region where the LET of the carbon ions is very high, near the end of the carbon-ion path (i.e., in the Bragg peak region). In contrast, in the "plateau" region of the depth-dose distribution of ions (i.e., in the normal tissue region), in which the LET is significantly lower, this generation of molecular oxygen is insignificant. Under FLASH irradiation, it is shown that this early generation of O2 extends evenly over the entire irradiated tumor volume, with concentrations estimated to be several orders of magnitude higher than the oxygen levels present in hypoxic tumor cells. Theoretically, these results indicate that FLASH radiotherapy using carbon ions would have a markedly improved therapeutic ratio with greater toxicity in the tumor due to the generation of oxygen at the spread-out Bragg peak.

Low Energy Beta Emitter Measurement: A Review

The detection and monitoring systems of low energy beta particles are of important concern in nuclear facilities and decommissioning sites. Generally, low-energy beta-rays have been measured in systems such as liquid scintillation counters and gas proportional counters but time is required for pretreatment and sampling, and ultimately it is difficult to obtain a representation of the observables. The risk of external exposure for low energy beta-ray emitting radioisotopes has not been significantly considered due to the low transmittance of the isotopes, whereas radiation protection against internal exposure is necessary because it can cause radiation hazard to into the body through ingestion and inhalation. In this review, research to produce various types of detectors and to measure low-energy beta-rays by using or manufacturing plastic scintillators such as commercial plastic and optic fiber is discussed. Furthermore, the state-of-the-art beta particle detectors using plastic scintillators and other types of beta-ray counters were elucidated with regard to characteristics of low energy beta-ray emitting radioisotopes. Recent rapid advances in organic matter and nanotechnology have brought attention to scintillators combining plastics and nanomaterials for all types of radiation detection. Herein, we provide an in-depth review on low energy beta emitter measurement.

Linear energy transfer dependence of transient yields in water irradiated by 150 keV – 500 MeV protons in the limit of low dose rates

FLASH radiotherapy is a new irradiation method in which large doses of ionizing radiation are delivered to tumors almost instantly (a few milliseconds), paradoxically sparing healthy tissue while preserving anti-tumor activity. Although this technique is primarily studied in the context of electron and photon therapies, proton delivery at high dose rates can also reduce the adverse side effects on normal cells. So far, no definitive mechanism has been proposed to explain the differences in the responses to radiation between tumor and normal tissues. Given that living cells and tissues consist mainly of water, we set out to study the effects of high dose rates on the radiolysis of water by protons in the energy range of 150 keV – 500 MeV (i.e., for linear energy transfer (LET) values between ∼72.2 and 0.23 keV/μm, respectively) using Monte Carlo simulations. To validate our methodology, however, we, first, report here the results of our calculations of the yields (G values) of the radiolytically produced species, namely the hydrated electron ([Formula: see text]), •OH, H•, H2, and H2O2, for low dose rates. Overall, our simulations agree very well with the experiment. In the presence of oxygen, [Formula: see text] and H• atoms are rapidly converted into superoxide anion or hydroperoxyl radicals, with a well-defined maximum of [Formula: see text] at ∼1 μs. This maximum decreases substantially when going from low-LET 500 MeV to high-LET 150 keV irradiating protons. Differences in the geometry of the proton track structure with increasing LET readily explain this diminution in [Formula: see text] radicals.

The low dose effects of human mammary epithelial cells induced by internal exposure to low radioactive tritiated water

Tritium is an important radioactive waste which needs to be monitored for radiation protection. Due to long biological half-life of organically bound tritium (OBT), the adverse consequence caused by chronic exposure of tritiated water (HTO) attracts concern. In this study, fibroblast cells were exposed to 2 × 10⁶ Bq/ml HTO to investigate the cellular behaviors. The dose relationship of survival fraction and γH2AX foci was a "U-shaped" curve. And the results of γH2AX intensity produced by ICCM, which was obtained from different doses, demonstrated bystander signal accounted for the protective effects induced by intermediate dose of 100 mGy. The comparison of temporal kinetics and spatial dynamics of DNA repair between tritium β-rays and γ-rays showed longer time was need for the dephosphorylation of H2AX protein after HTO exposure. It indicated complex cluster DSBs induced by tritium β-rays at the low dose impaired efficient recovery of DNA damage, which bear responsibility for the persistence of residual foci after low dose expsoure. It suggests after exposed to low dose radiation cells prefer to eliminate damage population to avoid DNA damage increasing the mutation potential.

Low linear energy transfer radiolysis of supercritical water at 400 °C: in situ generation of ultrafast, transient, density-dependent "acid spikes"

There is growing interest in the radiation chemistry of supercritical water (SCW), as its use as a coolant in a nuclear reactor (Generation IV) is the logical evolution of the current (Generation III or less) water-cooled reactors. However, current knowledge about the potential effects of water radiolysis in a Gen-IV supercritical water-cooled reactor (SCWR) is incomplete. In this work, Monte Carlo track chemistry simulations of the low linear energy transfer (LET) radiolysis of SCW (H2O) at 400 °C are used in combination with a spherical "spur" model to study the effect of water density on the in situ radiolytic formation of H3O+ ions and the corresponding abrupt, transient, highly acidic pH response ("acid spikes") that is observed immediately after irradiation. The magnitude and duration of this acidic pH effect depend on the water density in the considered range of 0.15-0.6 g cm-3. It is strongest at times less than a few tens of picoseconds with the pH remaining nearly constant at ∼1.6 and 1.9 for the highest ("liquid-like") and lowest ("gas-like") density, respectively. At longer times, the pH gradually increases for all densities and finally reaches a constant value corresponding to the non-radiolytic, pre-irradiation concentration of H3O+, due to the autoprotolysis of water. Our results show that the lower the density of the water, the longer the time required to reach this constant value, ranging from ∼50 ns at 0.6 g cm-3 (pH ∼ 5.6) to ∼1 μs at 0.15 g cm-3 (pH ∼ 8.5). The generation of these highly acidic pH fluctuations around the "native" radiation tracks, though local and transient, raises questions about the potential implications of this effect in proposed Gen-IV SCW-cooled reactors regarding corrosion and degradation of materials.

Self-radiolysis of tritiated water. 4. The scavenging effect of azide ions (N3−) on the molecular hydrogen yield in the radiolysis of water by 60Co γ-rays and tritium β-particles at room temperature

The effect of the azide ion N₃⁻ on the yield of molecular hydrogen in water irradiated with ⁶⁰Co γ-rays (∼1 MeV Compton electrons) and tritium β-electrons (mean electron energy of ∼7.8 keV) at 25 °C is investigated using Monte Carlo track chemistry simulations in conjunction with available experimental data. N₃⁻ is shown to interfere with the formation of H₂ through its high reactivity towards hydrogen atoms and, but to a lesser extent, hydrated electrons, the two major radiolytic precursors of the H₂ yield in the diffusing radiation tracks. Chemical changes are observed in the H₂ scavengeability depending on the particular type of radiation considered. These changes can readily be explained on the basis of differences in the initial spatial distribution of primary radiolytic species (i.e., the structure of the electron tracks). In the “short-track” geometry of the higher “linear energy transfer” (LET) tritium β-electrons (mean LET ∼5.9 eV nm⁻¹), radicals are formed locally in much higher initial concentration than in the isolated “spurs” of the energetic Compton electrons (LET ∼0.3 eV nm⁻¹) generated by the cobalt-60 γ-rays. As a result, the short-track geometry favors radical–radical reactions involving hydrated electrons and hydrogen atoms, leading to a clear increase in the yield of H₂ for tritium β-electrons compared to ⁶⁰Co γ-rays. These changes in the scavengeability of H₂ in passing from tritium β-radiolysis to γ-radiolysis are in good agreement with experimental data, lending strong support to the picture of tritium β-radiolysis mainly driven by the chemical action of short tracks of high local LET. At high N₃⁻ concentrations (>1 M), our H₂ yield results for ⁶⁰Co γ-radiolysis are also consistent with previous Monte Carlo simulations that suggested the necessity of including the capture of the precursors to the hydrated electrons (i.e., the short-lived “dry” electrons prior to hydration) by N₃⁻. These processes tend to reduce significantly the yields of H₂, as is observed experimentally. However, this dry electron scavenging at high azide concentrations is not seen in the higher-LET ³H β-radiolysis, leading us to conclude that the increased amount of intra-track chemistry intervening at early time under these conditions favors the recombination of these electrons with their parent water cations at the expense of their scavenging by N₃⁻.

The effect of the azide ion on the yield of molecular hydrogen in water irradiated with ⁶⁰Co γ-rays and tritium β-electrons at 25 °C is investigated using Monte Carlo track chemistry simulations.

Radiolytic Hydrogen Production in the Subseafloor Basaltic Aquifer

Hydrogen (H2) is produced in geological settings by dissociation of water due to radiation from radioactive decay of naturally occurring uranium ((238)U, (235)U), thorium ((232)Th) and potassium ((40)K). To quantify the potential significance of radiolytic H2 as an electron donor for microbes within the South Pacific subseafloor basaltic aquifer, we use radionuclide concentrations of 43 basalt samples from IODP Expedition 329 to calculate radiolytic H2 production rates in basement fractures. The samples are from three sites with very different basement ages and a wide range of alteration types. U, Th, and K concentrations vary by up to an order of magnitude from sample to sample at each site. Comparison of our samples to each other and to the results of previous studies of unaltered East Pacific Rise basalt suggests that significant variations in radionuclide concentrations are due to differences in initial (unaltered basalt) concentrations (which can vary between eruptive events) and post-emplacement alteration. However, there is no clear relationship between alteration type and calculated radiolytic yields. Local maxima in U, Th, and K produce hotspots of H2 production, causing calculated radiolytic rates to differ by up to a factor of 80 from sample to sample. Fracture width also greatly influences H2 production, where microfractures are hotspots for radiolytic H2 production. For example, H2 production rates normalized to water volume are 190 times higher in 1 μm wide fractures than in fractures that are 10 cm wide. To assess the importance of water radiolysis for microbial communities in subseafloor basaltic aquifers, we compare electron transfer rates from radiolysis to rates from iron oxidation in subseafloor basalt. Radiolysis appears likely to be a more important electron donor source than iron oxidation in old (>10 Ma) basement basalt. Radiolytic H2 production in the volume of water adjacent to a square cm of the most radioactive SPG basalt may support as many as 1500 cells.

A quantitative model of water radiolysis and chemical production rates near radionuclide-containing solids

We present a mathematical model that quantifies the rate of water radiolysis near radionuclide-containing solids. Our model incorporates the radioactivity of the solid along with the energies and attenuation properties for alpha (α), beta (β), and gamma (γ) radiation to calculate volume normalized dose rate profiles. In the model, these dose rate profiles are then used to calculate radiolytic hydrogen (H2) and hydrogen peroxide (H2O2) production rates as a function of distance from the solid-water interface. It expands on previous water radiolysis models by incorporating planar or cylindrical solid-water interfaces and by explicitly including γ radiation in dose rate calculations. To illustrate our model's utility, we quantify radiolytic H2 and H2O2 production rates surrounding spent nuclear fuel under different conditions (at 20 years and 1000 years of storage, as well as before and after barrier failure). These examples demonstrate the extent to which α, β and γ radiation contributes to total absorbed dose rate and radiolytic production rates. The different cases also illustrate how H2 and H2O2 yields depend on initial composition, shielding and age of the solid. In this way, the examples demonstrate the importance of including all three types of radiation in a general model of total radiolytic production rates.

Yields of H2 and hydrated electrons in low-LET radiolysis of water determined by Monte Carlo track chemistry simulations using phenol/N2O aqueous solutions up to 350 °C

The effect of temperature on the yields of H₂ and hydrated electrons in the low linear energy transfer radiolysis of water has been modeled by Monte Carlo track chemistry simulations using phenol/N₂O aqueous solutions from 25 up to 350 °C.

Acid spike effect in spurs/tracks of the low/high linear energy transfer radiolysis of water: potential implications for radiobiology

Monte Carlo track chemistry simulations have been used to calculate the yields of hydronium ions that are formed within spurs/tracks of the low/high linear energy transfer radiolysis of pure, deaerated water during and shortly after irradiation.

Revisited water radiolysis at elevated pH by accounting O3˙− kinetics at low and high LET

Recordings of O₃˙⁻ formation and decays under low and high LET radiations and at pH 13.2 allowed revisiting the rate constants of its reactions with O₂˙⁻ and HO₂⁻.