Carlson, Brett L.

Abstract Background Meningiomas lack effective therapies for recurrent or aggressive disease. NF2 inactivation, commonly in meningioma, may sensitize tumors to FAK inhibition and interact with MAPK signaling. This study investigated the therapeutic activity of defactinib (FAK inhibitor) and avutometinib (RAF/MEK inhibitor) alone and in combination in preclinical meningioma models. Methods MAPK-altered meningioma cell lines, IOMM-Lee (NF2 wild-type, BRAF p.V600E) and CH157-MN (NF2 mutant, NRAS p.Q61K), BenMen-1 (NF2 mutant, MAPK wild-type), and three patient-derived NF2 mutant cell lines were cultured in matrigel and treated with defactinib and avutometinib in 3D cell viability assays. In vivo, flank and intracranial xenograft models of IOMM-Lee and CH157-MN were treated with VS-4718 (FAK inhibitor, superior pharmokinetics in mice), avutometinib, or combination. Tumor growth, survival, and pharmacokinetic/pharmacodynamic properties were assessed. Results In vitro, combination therapy showed synergistic or additive effects in CH157-MN and IOMM-Lee cells, respectively. Defactinib alone demonstrated activity in NF2-mutant patient-derived and BenMen-1 cells, while avutometinib did not. In vivo, avutometinib monotherapy suppressed tumor growth and extended survival in both flank and intracranial xenograft models, with evidence of favorable pharmacokinetic properties and target pathway inhibition in intracranial tumors. Combination therapy provided enhanced and sustained control of flank CH157-MN meningioma compared to avutometinib alone but did not significantly improve survival over avutometinib alone in intracranial models. Conclusions Avutometinib demonstrated robust anti-tumor effects in flank and intracranial models of MAPK-altered meningioma. Combination benefits were context dependent. Defactinib activity in NF2-mutant patient-derived cells without targetable MAPK-kinase alterations highlights the importance of genomic biomarkers to guide personalized strategies.

Abstract Radiation is a cornerstone of glioblastoma (GBM) treatment, however, the majority of tumors progress within the radiation field due to inherent radiotherapy (RT) resistance. The integration of highly potent radiosensitizers, such as the ATM inhibitors AZD1390 or WSD0628, could reverse resistance and significantly improve local control of these tumors. However, the achievable brain tumor exposure of these inhibitors may be limited by drug exposure in skin and mucosa, leading to enhanced radiation toxicity. To support the clinical development of WSD0628, we developed pharmacokinetic (PK)-efficacy models to help interpret ongoing PK analyses. A model based on in vitro cell studies was based on the in vitro observations that 30 nM WSD0628 provided maximum ATM inhibition and the extent of radiosensitization was directly related to the duration of drug incubation after irradiation, up to a maximum of 24 hours. The unbound WSD0628 concentration, measured by rapid equilibrium dialysis, was 66% of the total concentration in cell culture media, and was used to define an optimal radiosensitizing target of 20 nM free WSD0628 for 4 to 24 hours. Previous PK analyses showed that the free, unbound fraction (fu) of WSD0628 in plasma and brain tumor tissue are 0.02 and 0.054, respectively, and the brain tumor-to-plasma ratio is 0.32 in GBM43 patient-derived xenografts (PDXs). From these data, a total drug level of 1160 nM in plasma is predicted to achieve unbound drug levels above the 20 nM WSD0628 target unbound concentration. To validate this prediction, parallel in vivo dose-ranging studies were performed in orthotopic PDXs. Mice with orthotopic GBM43 were randomized to treatment with RT (8 Gy x1) alone or in combination with a range of WSD0628 doses. RT combined with 0.25 mg/kg and 1 mg/kg WSD0628 had no impact on survival, compared to RT alone, while median survival increased by 1.3-fold for RT combined with 2.5 mg/kg, 1.7-fold with 5 mg/kg, and 3.2-fold with 10 mg/kg WSD0628. A similar dose-response was observed in an M12 melanoma brain metastasis PDX. Based on PK modeling of these drug doses, 2.5 mg/kg dosing will maintain total WSD0628 plasma levels above 1160 nM for 11.9 hours, 5 mg/kg for 23.6 hours, and 10 mg/kg for 40 hours. Taken together, we have developed a model to predict efficacious dosing of a brain tumor based on the free drug hypothesis, which allows integration of multiple aspects of in vitro modeling with measured drug levels in animal models to predict total plasma levels associated with robust sensitizing effects. Assuming similar tissue to plasma partitioning in human GBM, this model could be directly applied to interpret the ongoing Phase 1 PK analysis of WSD0628 in recurrent GBM. Citation Format: Ann Catherine Mladek, Juhee Oh, Sneha Rathi, Lily Liu, Danielle M. Burgenske, Brett L. Carlson, Katrina K. Bakken, Lauren L. Ott, Zeng Hu, Wei Zhong, William F. Elmquist, Jann N. Sarkaria. Modeling pharmacokinetics and efficacy of the ATM inhibitor WSD0628 in GBM and melanoma metastasis PDX models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 4629.

Abstract Therapeutic resistance limits the efficacy of radiation therapy in many tumors and provides a compelling rationale to develop novel treatments to enhance radiation sensitivity. DNA-dependent protein kinase (DNA-PK) is a major mediator of DNA repair following radiation, and DNA-PK inhibitors are effective radiosensitizers. WNC0901 is a potent DNA-PK inhibitor in both a cell-free kinase assay (IC50 = 0.073 nM) and a cell-based assay (IC50 = 76 nM). WNC0901 has a 30-fold specificity for DNA-PK relative to other PI3K-related kinases. In a broader kinome screen (363 kinases), 1 μM WNC0901 resulted in >25% inhibition of only five other kinases. There is robust inhibition of radiation-induced DNA-PK autophosphorylation (Ser-2056), as analyzed by Western blotting, with maximal effect at 300 nM WNC0901 in A549 (lung carcinoma), U2OS (osteosarcoma), and HT29 (colorectal) cells. The efficacy of WNC0901 as a radiosensitizer was evaluated in clonogenic assays across these same three radioresistant tumor models. Following treatment with graded concentrations of WNC0901 (0-1000 nM) combined with 2.5 Gy radiation, maximal effects were observed at a 300 nM concentration in A549 (88-fold reduction in relative clonogenic survival compared to RT alone, p<0.001), U2OS (5.8-fold decrease, p=0.002), and HT29 (59-fold decrease, p=0.008) In a full radiation clonogenic, the sensitizer enhancement ratio at 10% survival (SER10) was 3.8 (A549), 2.3 (U2OS), and 1.8 (HT29). In vitro metabolism studies have identified demethylated WNC0901 (WNC0901-M) as a major metabolite. Similar to the parent drug, WNC0901-M is a potent DNA-PK inhibitor with maximal radiosensitizing effects of the metabolite observed at 300nM with SER10 of 2.4 (A549), 1.9 (U2OS), and 1.8 (HT29). The in vivo efficacy of 50 mg/kg WNC0901 combined with 8 Gy was evaluated and compared to another DNA-PK inhibitor, 50 mg/kg peposertib, in A549 tumor heterotopic xenografts. Compared to placebo (median time to endpoint 63 days) or RT alone (median 72 days), the combination of WNC0901 and RT significantly delayed tumor regrowth (median 169 days; p=0.002 compared to RT). Interestingly, peposertib + RT was ineffective (median 83 days; p=0.332 compared to RT). In a second study, similarly robust sensitizing effects of WNC0901 was observed in HT29 xenografts (placebo - median 45 days; RT - median 75 days; RT + WNC0901 - median >110 days; p=.03 relative to RT). Together, these findings demonstrate the robust radiosensitizing effects of WNC0901 in radioresistant tumors and provide a rationale for continued development of this drug. Citation Format: Jessica L. Abraham, Ann C. Mladek, Sonia Jain, Shiv K. Gupta, Paul A. Decker, Matthew L. Kosel, Katrina K. Bakken, Brett L. Carlson, Lauren L. Ott, Danielle M. Burgenske, Jeanette E. Eckel-Passow, Wei Zhong, Jann N. Sarkaria. WNC0901 is a novel DNA-PK inhibitor and potent radiosensitizer of otherwise radioresistant solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6608.

Abstract Background Murine double minute 2 (MDM2) inhibitors reactivate wild-type p53 and are a promising therapy for glioblastoma, IDH-wildtype (GBM). Brigimadlin is a highly potent MDM2 inhibitor being tested in a phase 0/1 clinical trial in combination with radiation in GBM. Methods Brigimadlin pharmacokinetics, pharmacodynamics, and efficacy were evaluated in GBM patient-derived xenografts (PDXs). Results In vitro, brigimadlin impaired viability in TP53 wild-type GBM with an IC50 of 0.8-6.6 nmol/L, but sensitivity did not correlate with MDM2 amplification. In vivo, MDM2 amplification was highly correlated with efficacy. In subcutaneous PDXs, 1 or 2 mg/kg brigimadlin dosed weekly was highly effective in 2 MDM2-amplified PDXs. At 2 mg/kg, brigimadlin delayed tumor regrowth by >5-fold in the MDM2-amplified PDXs compared to 1.5-fold in a non-amplified PDX. In orthotopic PDXs, efficacy was more limited, but 2 mg/kg brigimadlin enhanced the response to fractionated radiation in MDM2-amplified PDXs. Consistent with blood-brain barrier efflux limiting drug distribution, 2 mg/kg brigimadlin extended survival by >5-fold in an MDM2-amplified orthotopic PDX established in Rag1−/−Abcb1a−/− Abcg2−/− mice. In pharmacodynamic studies, p53 target genes were upregulated at both subtherapeutic and therapeutic dose levels, and the extent of activation did not correlate with MDM2 status. Concentrations of brigimadlin in tumor tissue were approximately 10-fold higher in MDM2-amplified tumors, and intracellular drug levels directly correlated with drug-dependent MDM2 upregulation, suggesting target binding affects drug accumulation. Conclusions Brigimadlin is highly effective in MDM2-amplified GBM when adequate drug levels are achieved in tumor tissue. MDM2 amplification impacts both treatment efficacy and intratumoral drug accumulation.

Radiation exposure to the eye during cancer treatment can lead to ocular radiation injury (ORI), a devastating condition that can have a profound and permanent impact on vision-related quality of life. Rodent models do not have adequate ocular anatomy to accurately simulate human ORI, and modeling in non-human primates is limited by logistical and ethical concerns. To improve future translational research investigating ways to treat or prevent ORI, we developed protocols for a tree shrew model of ORI. Northern tree shrews (Tupaia belangeri) were obtained by our laboratory. Custom housing and handling methods were developed, including custom body suits to maintain the tree shrew’s body temperature during procedures. Radiation delivery was optimized to accurately deliver radiation, and imaging was performed to observe fundus changes from ORI. Optimization of tree shrew handling, housing, anesthesia approaches, radiation delivery, and clinically-relevant ocular imaging permitted successful induction and assessment of ORI in tree shrews. With these protocols, tree shrews can be used as a highly relevant model organism with key anatomic features similar to humans to study ORI.

Abstract BACKGROUND High-grade and recurrent meningiomas frequently demonstrate aggressive behavior and therapeutic resistance, with limited treatment options. Dysregulation of the MAPK and FAK pathways drive meningioma progression, but compensatory activation between these pathways could limit single agent efficacy. Recently, a clinical trial (Alliance A071401) showed encouraging efficacy of single-agent FAK inhibition in recurrent/progressive meningioma (Brastianos et al, JCO 2023). This preclinical study aimed to evaluate the therapeutic potential of dual RAF/MEK (avutometinib) and FAK (VS-4718) inhibition in suppressing tumor growth and improving survival in intracranial meningioma models. METHODS Orthotopic intracranial models were established by injecting IOMM-Lee (Fluc-mCherry) or CH157-MN (eGFP-LUC2) cells into nu/nu mice. Mice were randomized into four groups: vehicle, VS-4718 (murine formulation of defactinib, 50 mg/kg, BID), avutometinib (0.3 mg/kg, QD), or combination therapy. Treatments were administered for a 4-week period. Tumor growth was monitored via bioluminescent imaging (BLI) at baseline, midpoint, and study endpoint. Pharmacokinetic (PK) and pharmacodynamic (PD) effects were assessed by intratumoral drug concentrations and Western blotting for p-ERK and p-AKT, respectively. RESULTS In the NF2-mutant CH157-MN model, avutometinib increased median survival from 12.5 to 20.5 days (p=0.001), while combination therapy extended survival further to 22 days (p<0.0001 vs vehicle). Similarly, in the NF2-wildtype IOMM-Lee model, avutometinib raised survival from 27.5 to 33 days (p=0.0237), with combination therapy reaching 34.5 days (p=0.0057 vs vehicle). VS-4718 monotherapy did not extend survival in both models. BLI demonstrated significant inhibition of tumor growth in both avutometinib and combination groups in both tumor models. PK analysis revealed Kp values of 0.26–0.37 for avutometinib and 0.46–0.50 for VS-4718 in orthotopic tumor tissue. Furthermore, combination therapy lowered p-ERK in tumors (p=0.001) compared to vehicle. CONCLUSION Dual RAF/MEK and FAK inhibitor therapy blocks MAPK activation and enhances survival in orthotopic meningioma models, supporting further clinical evaluation for aggressive meningiomas.

Abstract Failure to repair radiation (RT)-induced DNA damage results in cell lethality, and combined treatment with DNA repair inhibitors can markedly enhance tumor control achieved with radiation. These inhibitors also can potentiate acute RT toxicities by suppressing DNA repair. Capitalizing on this normal tissue radiosensitizing effect, we determined effective radiosensitizing dosing regimens in mice for the ATM inhibitor WSD0628 and the DNAPK inhibitor WNC0901 in mice. Drugs were administered via oral gavage prior to each RT dose, and RT was delivered using an anterior/posterior 225 kVp beam. In an initial iteration of this model, various combinations of WSD0628 and RT were administered to cohorts of mice (n=5/group) and body weight was followed for 2 weeks. The resulting data were modeled to define the total dose of radiation estimated to cause 50% of mice to lose >20% body weight (D50). There was an inverse relationship between the WSD0628 and D50 doses: 0 mg/kg WSD0628/24.0 Gy (95% confidence interval (CI) 24.5–24.7); 0.25 mg/kg/19.2 Gy (95% CI 16.7–21.8); 1 mg/kg/13.7 Gy (95% CI 13.2–14.2); 2.5 mg/kg/11.0 Gy (95% CI 10.5–11.5); 5 mg/kg/11.3 Gy (95% CI 6.7–15.9) and 7.5 mg/kg/7.8 Gy (95% CI 7.8–7.9). Histological review of tongues from animals treated with 15 Gy RT (5Gy x 3) + 7.5 mg/kg WSD0628 demonstrated epithelial denudation and acute inflammation consistent with oral mucositis, while these features were not seen with the same dose as RT alone. Similar WSD0628 doses were evaluated in combination with 8 Gy supratentorial RT (via lateral beams) in orthotopic GBM43 PDX tumors. By treatment arm, the median time to a moribund state was: sham (29 days (d)), RT (39d), 1 mg/kg (42d), 2.5 mg/kg (49d), 5 mg/kg (65d), and 10 mg/kg (123d, overall p<0.0001). A second mucositis model was used for evaluation of WNC0901. Cohorts of 5 mice/group were treated with a set dose of RT (4Gy x 3) to the oral cavity alone and in combination with various doses of WNC0901, and the fraction of mice with >20% weight loss within 2 weeks was determined. The results of this study are as follows: placebo (0 of 5 mice with >20% weight loss); 15 mg/kg (3 of 5 mice); 25, 50, or 100 mg/kg (5 of 5 mice). Based on these data, we defined 25 mg/kg WNC0901 as the minimally effective dose and predicted that 50 mg/kg would provide robust radiosensitization. Consistent with this prediction, co-treatment of B16 melanoma flank tumor xenografts with RT (8Gy x 1) and WNC0901 (50 mg/kg) more than doubled the median tumor control duration (median 42 [range 24-45] days; time from implantation for tumor to exceed 1500 mm3) as compared to RT alone (24 [13-30] days) or sham treatment (17 [13-17] days; overall p<0.0001). This study describes how a radiation-induced oral mucositis model can be used to identify effective radiosensitizing regimens faster and with fewer resources than relying on tumor control studies alone. Citation Format: Danielle Burgenske, Darwin Garcia, Jiayan Le, Wenjuan Zhang, Sneha Rathi, Katrina K. Bakken, Lauren L. Ott, Zeng Hu, Brett L. Carlson, Ann Mladek, Jessica L. Abraham, Paul A. Decker, Matthew L . Kosel, Jeanette E. Eckel-Passow, William F. Elmquist, Wei Zhong, Jann N. Sarkaria. Radiation-induced oral mucositis to rapidly determine effective radiosensitizing dosing regimens for ATM and DNAPK inhibitors [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2025 Oct 22-26; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2025;24(10 Suppl):Abstract nr B090.

Abstract Purpose: Navtemadlin is a potent small-molecule inhibitor of MDM2, which has completed a phase 0 window-of-opportunity study in glioblastoma (GBM). To optimally interpret the clinical data, a detailed analysis of navtemadlin pharmacokinetics (PK), pharmacodynamics, and efficacy was performed in GBM patient-derived xenografts (PDX). Experimental Design: Response to navtemadlin was characterized in vitro and in vivo in GBM PDXs with and without MDM2 amplification. Efficacy in vivo was integrated with measured plasma and intra-tumoral drug levels to develop a translational PK/efficacy model comparing exposure effective in PDX to exposure achieved in phase 0 patient samples. Results: In vitro, navtemadlin showed robust on-target activity in TP53–wild-type GBM. In vivo efficacy strongly correlated with MDM2 amplification status. In subcutaneous PDXs, navtemadlin significantly extended survival when dosed at 25 mg/kg in an MDM2-amplified PDX compared with 100 mg/kg in a non-amplified PDX. Central nervous system distribution was limited by blood–brain barrier efflux (Kp_brain = 0.009). In an MDM2-amplified orthotopic PDX model, navtemadlin was ineffective at 100 mg/kg; when established in mice with deficient blood–brain barrier efflux (Rag−/−Abcb1a−/−Abcg2−/−), 25 mg/kg doubled survival. A tumor PK/efficacy model was built to define target exposure for efficacy in GBM, using the effective 25 mg/kg dose. Modeled exposures exceeded this threshold in three (of 16) tumor samples from phase 0 study patients at the 240 mg dose level. Conclusions: Navtemadlin efficacy was highly dependent on adequate brain penetration. Our translational PK/efficacy model suggests that the minimum effective tumor exposures were achieved only in a minority of patients with GBM.

Temozolomide (TMZ) is the main chemotherapy for GBM, but resistance develops in most patients, highlighting the need for sensitizing strategies. PARP inhibitors (PARPi), which disrupt DNA repair, are being investigated as TMZ-sensitizing agents. Here we evaluated the efficacy and explored molecular mechanisms of pamiparib-mediated TMZ sensitization. In vitro, low concentrations of pamiparib (0.1-1.0 μM) synergized with TMZ and enhanced DNA damage, particularly during the S- and G2/M phases. In vivo, pamiparib monotherapy (0.75 or 3 mg/kg, twice daily) was ineffective, however, TMZ afforded survival benefit in orthotopic patient-derived xenograft (PDX) models (GBM12, GBM22, and GBM84) of MGMT-methylated primary GBM. Pamiparib combined with TMZ (25 or 50 mg/kg daily) prolonged survival in GBM12 [median survival 272 vs. 114 days with TMZ (P=0.02), 30 days with placebo (P<0.001)] and GBM22 (median survival 88 vs. 60 days with TMZ (P=0.05), 26 days with placebo, (P<0.001)] but not in GBM84 [median survival 355 vs. 404 days with TMZ (P=0.44), 37 days with placebo (P<0.001)]. These findings align with prior studies indicating that PARPi can potentiate TMZ efficacy in a subset of primary GBM models sensitive to TMZ. TMZ induces cytotoxic DNA adducts, N7MeG, N3MeA, and O6MeG. While N7MeG and N3MeA are repaired by base-excision repair (BER), MGMT removes O6MeG. Suppressed MGMT expression, due to promoter methylation, drives TMZ sensitivity in ∼40% of primary GBM cases. In MGMT-methylated tumors, O6MeG-thymine mispairs trigger futile mismatch repair, causing replication stress. Although BER is PARP-dependent, BER inhibition did not replicate the TMZ-sensitizing effects of pamiparib. Beyond DNA repair, PARP regulates replication stress by recruiting translocases like SMARCAL1 and controlling endonucleases such as MRE11 and EXO1, protecting stressed forks and nascent DNA. We targeted these components using siRNA and small-molecule inhibitors to elucidate their roles in TMZ sensitization. Unexpectedly, silencing SMARCAL1 or MRE11, or inhibiting MRE11 with mirin, did not affect pamiparib-mediated sensitization in U251 cells. While EXO1 silencing attenuated DNA damage signaling, it had minimal impact on TMZ sensitivity or pamiparib-mediated sensitization. These results suggest that fork protection mechanisms may be epistatic to PARP but redundant for TMZ sensitivity. Alternatively, replication stress may be mitigated by lesion bypass via translesion synthesis (TLS), where replicative polymerases are replaced with low-fidelity polymerases. Consistent with this, silencing RAD18, involved in TLS, enhanced sensitivity to both TMZ and pamiparib, with the combination being more effective than either drug alone. In conclusion, pamiparib enhances TMZ efficacy in a subset of primary GBM. While further in vivo validation is needed, TLS deficiency may be a key determinant of pamiparib-mediated sensitization. Citation Format: Shiv K. Gupta, Ann C. Mladek, Sonia Jain, Katrina K. Bakken, Zeng HU, Brett L. Carlson, Danielle M. Burgenske, Jann N. Sarkaria. Pamiparib-induced replication stress augments the efficacy of temozolomide in GBM PDX models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2910.

Total body irradiation for bone marrow transplantation (BMT) conditioning is commonly used in immunologic research. Historically, cesium-137 (137Cs) irradiators were predominantly used as a radiation source for these studies. However, 137Cs has a moderate half-life (30 years), is highly water soluble, and poses significant environmental and radiological safety concerns. As a result, the Department of Energy is promoting the replacement of 137Cs irradiation systems with orthovoltage x-ray cabinet systems, which pose no such concerns. Because the physical characteristics of x-ray and 137Cs photon beams have slightly different energy deposition characteristics, the relatively biological effectiveness (RBE) for x-rays is slightly higher than 137Cs in cell survival studies. This study aims to compare the RBE of orthovoltage and 137Cs irradiators with regards to BMT efficiency. Total body irradiation was performed on C57BL6 mice using 137Cs or 320 kVp x-rays across six dose levels to establish a RBE for lethality. Cohorts of five mice were irradiated using split-dose radiation delivered four hours apart. Daily observations and body weights were captured over a 30-day period. Mice with poor body condition scores or weight loss ≥20% were euthanized. With x-rays, 50% lethality was observed ∼750 cGy while 137Cs required ∼850 cGy, which results in an RBE of 750 cGy/850 cGy = 0.88. Bone marrow ablation and subsequent engraftment efficacy was compared in a parallel study. Cohorts of five C57BL6 CD45.1 recipient mice received total body irradiation with an established single dose 1000 cGy 137Cs regimen or a range of x-ray doses (910-1030 cGy single-dose) and then were infused with CD45.2 donor bone marrow. Ten weeks after BMT, bone marrow, spleen, thymus, and peripheral blood were analyzed via multi-color flow cytometry and the percentage of CD45.1 and CD45.2-positive cells were analyzed to determine marrow engraftment efficiency across various immune compartments. A clear dose-response relationship with bone marrow ablation and engraftment was observed in the stem cell and progenitor compartments. LSK (Lineage- Sca-1+ c-Kit+) cells and other hematopoietic progenitors in the bone marrow were efficiently engrafted (>99%) except for the lowest dose of 910 cGy (around 98%). Similarly, in the spleen, B cells, myeloid, megakaryocyte/erythrocyte, and granulocyte/macrophage lineages were also efficiently engrafted similar to that of 137Cs. Developing thymocytes were efficiently replaced by all radiation regimens, while replacement of peripheral T cell populations varied but were similar to traditional 137Cs at the highest doses. Taken together, these data demonstrate comparable BMT efficiencies with orthovoltage irradiation after appropriate dose-optimization. Citation Format: Danielle M. Burgenske, Brett L. Carlson, Matthew J. Rajcula, Michael J. Shapiro, Virginia M. Shapiro, Jann N. Sarkaria. An in vivo comparison of 137Cs and orthovoltage irradiation for murine bone marrow transplantation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4684.