PURPOSE:Antibody-drug conjugates (ADC) harboring topoisomerase I (TOP1) inhibitor payloads have improved survival for patients with metastatic breast cancer. However, knowledge of ADC resistance mechanisms and potential impact on the sequential use of ADCs is limited. In this study, we report the incidence and characterization of TOP1 mutations arising in the setting of ADC resistance in metastatic breast cancer. EXPERIMENTAL DESIGN:Patients with metastatic breast cancer treated with ADCs with available posttreatment plasma-based genotyping were included. TOP1 mutation incidence, mutant allele frequency, and functional characterization were assessed, and incidence was compared with that in patients with metastatic breast cancer not receiving ADC treatment and in The Cancer Genome Atlas. RESULTS:Plasma-based genotyping identified distinct TOP1 mutations (S57C, R364H, W401C, and G359E) in 12.9% of patients (4/31) at the time of disease progression on ADC, compared with 0.7% (3/420) in non-ADC-treated patients with metastatic breast cancer and 0.5% in The Cancer Genome Atlas. The appearance of mutations was associated with clinical cross-resistance, as median duration on the first ADC was 455 versus 52 days for the second ADC. The functional characterization of three novel TOP1-mutant proteins demonstrated that all exhibited reduced enzymatic activity, attenuated covalent DNA binding, and resistance to TOP1 inhibitor ADC payloads SN38 and deruxtecan. CONCLUSIONS:We describe the recurrent emergence of functionally altered, resistance-associated TOP1 mutations in vivo under selective pressure from ADCs and the potential impact on mediating cross-resistance to sequential ADCs. TOP1 mutation may represent a biomarker of resistance in this setting, and additional work is needed to optimize biomarkers and ADC payload design to improve outcomes for the sequential use of ADCs. See related commentary by Gwin and Hurvitz, p. 1824.

Abstract Background: Pancreatic ductal adenocarcinoma (PDAC) has a 5-year survival rate of only 12%, with limited treatment options. Oncogenic KRAS mutations drive PDAC tumor progression, which occur in the majority of PDAC tumors. KRAS G12D is the most common KRAS mutation, found in 36% of PDAC cases, which is associated with the worst clinical outcomes. Mutant-selective KRAS G12D inhibitors (KRASi G12D), such as MRTX1133, have recently demonstrated initial responses in KRAS G12D-mutant preclinical models. However, mechanisms of adaptive resistance to KRAS G12D inhibition are largely under-investigated and novel therapeutic strategies to overcome drug resistance are urgently needed. Results: Adaptive RAS signaling reactivation was found to dampen KRAS G12D inhibition in a library of KRAS G12D PDAC cell lines, thereby minimizing drug efficacy. Quantitative downstream analyses of drug combination screening using a KRAS G12D cell line library further identified this adaption to KRAS inhibition was cell subtype specific. Fibroblast growth factor receptor (FGFRs) drove adaptive RAS reactivation specifically in mesenchymal PDAC cells, which are aggressive, and intrinsically resistant to KRASi. In contrast, epithelial growth factor receptor (EGFR) reactivated RAS G12D signaling in epithelial cells, which are sensitive to KRASi G12D. Genetic and pharmacological inhibition of FGFR1 or FGFR2 improved drug efficacy by decreasing FGFR-mediated RAS reactivation specifically in mesenchymal PDAC lines. Pharmacological blockade of EGFR enhanced drug efficacy by inhibiting EGFR-mediated RAS reactivation specifically in epithelial PDAC lines. Furthermore, low-concentration of a novel pan- RAS inhibitor, which inhibits both wild-type and mutant RAS, completely abrogated MAPK signaling and significantly improved efficacy, when combined with KRASi G12D, suggesting a pro-survival dependency on wild-type RAS as a compensatory mechanism in response to KRAS G12D inhibition. Conclusion: Upstream RTK-mediated adaptive RAS signaling re-activation in response to KRAS inhibition in PDAC is cell subtype specific. Complete inhibition of RAS signaling by overcoming adaptive RAS rebound improves efficacy of KRASi G12D. Citation Format: Qingxiang (Nick) Lin, Alvin Morales, Haley Barnes, Ryan Bruce Corcoran. Overcoming Cell Subtype-Specific Adaptive Resistance to KRAS G12D Inhibition in KRAS G12D-Mutant Pancreatic Cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pancreatic Cancer Research; 2024 Sep 15-18; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2024;84(17 Suppl_2):Abstract nr C012.

Supplementary Figure S1 shows IC50 values of KRASG12C inhibitors for KRASG12C, NRASG12C, HRASG12C. Supplementary Figure S2 shows inhibition of NRASG12C by sotorasib in 293T and MOLT-4 cells. Supplementary Figure S3 shows effect of amino acid substitutions in NRASG12C or KRASG12C on signaling in response to sotorasib and adagrasib. Supplementary Figure S4 who's location of isoform-variable residues relative to Switch-II binders. Supplementary Figure S5 shows crystal structures of KRASG12C inhibitors and interactions with His95. Supplementary Figure S6 shows chemical structures of switch-II pocket-binding KRASG12C inhibitors. Supplementary Figure S7 shows structural model of sotorasib interaction with Leu95.

Abstract KRASG12C inhibitors, like sotorasib and adagrasib, potently and selectively inhibit KRASG12C through a covalent interaction with the mutant cysteine, driving clinical efficacy in KRASG12C tumors. Because amino acid sequences of the three main RAS isoforms—KRAS, NRAS, and HRAS—are highly similar, we hypothesized that some KRASG12C inhibitors might also target NRASG12C and/or HRASG12C, which are less common but critical oncogenic driver mutations in some tumors. Although some inhibitors, like adagrasib, were highly selective for KRASG12C, others also potently inhibited NRASG12C and/or HRASG12C. Notably, sotorasib was five-fold more potent against NRASG12C compared with KRASG12C or HRASG12C. Structural and reciprocal mutagenesis studies suggested that differences in isoform-specific binding are mediated by a single amino acid: Histidine-95 in KRAS (Leucine-95 in NRAS). A patient with NRASG12C colorectal cancer treated with sotorasib and the anti-EGFR antibody panitumumab achieved a marked tumor response, demonstrating that sotorasib can be clinically effective in NRASG12C-mutated tumors. Significance: These studies demonstrate that certain KRASG12C inhibitors effectively target all RASG12C mutations and that sotorasib specifically is a potent NRASG12C inhibitor capable of driving clinical responses. These findings have important implications for the treatment of patients with NRASG12C or HRASG12C cancers and could guide design of NRAS or HRAS inhibitors. See related commentary by Seale and Misale, p. 698. This article is featured in Selected Articles from This Issue, p. 695

Abstract Background: Alterations in the SPEN gene are rare in primary breast cancer (~3%, ref: TCGA). SPEN is a hormone-inducible transcriptional repressor with known functions in orchestrating X-inactivation in females. Additionally, SPEN has been implicated as a potential estrogen receptor co-repressor and molecular partner of NCOR2 as well as epigenetic modifiers such as KMT2D and HDACs via C-terminal interactions. SPEN loss-of-function is associated with tamoxifen resistance in preclinical models. However, the landscape of SPEN alterations in advanced breast cancer remains poorly described. The primary objective of this study was to evaluate the frequency and different types of SPEN alterations in metastatic breast cancer (MBC). Methods: A query was performed on all patients at an academic institution with available genomic testing from Guardant360, a next-generation sequencing cell-free DNA (cfDNA) assay, evaluating 500 genes in total. Patients with MBC were identified, and specific alterations in SPEN as well as co-alterations were extracted. Silent and germline mutations were excluded from analysis. Retrospective review was conducted to determine clinicopathologic characteristics such as age, hormone and HER2 receptor status, menopausal status, and therapy at time of the detected mutation. Results: Among 366 patients with MBC and available cfDNA, 8.5% (n=31) had an alteration detected in the SPEN gene. Median age of patients with SPEN mutations was 66 years (range 33-92). A majority of patients (90%) had HR+ disease (n=28/31), with HR-/HER2+ disease and TNBC additionally representing 3% (n=1/31) and 6% (n=2/31) of cases, respectively. SNVs were most frequent, comprising 89% (n=42/47) of all alterations. Indels were additionally identified in 11% (n=5/47), and no CNVs or fusions were noted. Nonsense and frameshift mutations represented 8/47 (17%) and 2/47 (4%) of all alterations respectively. No recurrent mutational hotspots were detected. The most common co-alterations in descending order were PIK3CA, TP53, ERBB2, ARID1A, MDM4, PPM1D, NF1, DNMT3A, ATM, and ESR1. Association between SPEN mutations and clinical outcomes will be presented at the meeting. Conclusions: SPEN mutations were detected mostly in patients with HR+/HER2- MBC and at a higher frequency than reported in primary disease via TCGA. SNVs were common, though it remains unclear whether these comprise driver versus passenger mutations. While no mutational hotspots were identified, several novel loss-of-function truncating mutations were identified, which warrant further validation of functional and clinical significance. SPEN alterations in metastatic breast cancer Alterations detected by cell-free DNA sequencing in patients with metastatic breast cancer and annotated by type of alteration and receptor subtype. Citation Format: Charles Dai, Haley Barnes, Arielle Medford, Annika Putur, Jennifer Keenan, Beverly Moy, Seth Wander, Ryan Corcoran, Aditya Bardia. Detection of SPEN mutations in advanced breast cancer by circulating tumor cell-free DNA [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-13-02.

Abstract Background: Antibody-drug conjugates (ADCs) have improved survival for patients with metastatic breast cancer (MBC). In patients with HER2 negative MBC, two ADCs have FDA approval: sacituzumab govitecan (SG) and trastuzumab deruxtecan (T-DXd), both with topoisomerase-I (topo-I) inhibitor payloads. Given the many other ADCs in development, there is great interest in using ADC after ADC to maximize treatment benefit for patients. However, there is limited understanding regarding resistance mechanisms to ADCs and impact on ADC sequencing. We conducted a translational study to address this, and here we report the incidence of TOP1 mutations and clinical impact on ADC sequencing. Methods: All patients with MBC treated with ADCs at a large academic medical institution (Massachusetts General Hospital) who had comprehensive plasma-based genotyping (500 gene GuardantOMNI panel) were included. Since both SG and T-DXd have topo-I inhibitor-based payloads, we particularly focused on TOP1 mutations, variant allele frequency (VAF), and germline/somatic characterization. Incidence of TOP1 mutations in the ADC cohort was compared to The Cancer Genome Atlas (TCGA). Clinical “cross-resistance” was defined as progressive disease (PD) as best response to second ADC (ADC2) or treatment time on ADC2 of less than 60 days. Results: Based on comprehensive plasma-based genotyping we identified 4 distinct TOP1 mutations: S57C, R364H, W401C, G359E, at a frequency of 6.0% (4/67) at the time of disease progression on ADC compared to a frequency of 0.5% described in primary breast cancer in TCGA. Two of the amino acids found to be mutated are known to form direct interactions with the DNA backbone (G359) or the topoisomerase inhibitor itself (R364). For clinical resistance, one patient was briefly on ADC2 but stopped after 1 dose for toxicity; among the other 3 patients, two had cross-resistance to ADC after ADC, with both ADCs containing topo-I inhibitor payloads. Median duration on first ADC was 455 days compared to a median of 52 days for ADC2. Finally, one patient treated sequentially with 3 ADCs (all with topo-I inhibitor payloads) was found to have rising TOP1 VAF with progressive ADC treatments. Conclusion: This is the first report describing emergence of TOP1 mutations under selective pressure from ADCs and the impact on mediating cross-resistance to ADC after ADC with topo-I inhibitor payloads. Novel ADCs with alternative payloads may potentially be more effective when used sequentially after an ADC with a topo-I inhibitor. Further biomarker research is needed to optimize ADC sequencing for patients with TOP1 mutant MBC. Citation Format: Rachel Occhiogrosso Abelman, Haley Barnes, Arielle J. Medford, Annika Putur, Bogang Wu, Caroline Weipert, Geoffrey Fell, Laura M. Spring, Seth A. Wander, Beverly Moy, Andreas Varkaris, Dejan Juric, Leif Ellisen, Ryan Corcoran, Aditya Bardia. TOP1 mutations mediate cross resistance to ADCs in metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3888.

Although KRAS has long been considered undruggable, direct KRASG12C inhibitors have shown promising initial clinical efficacy. However, the majority of patients still fail to respond. Adaptive feedback reactivation of RAS-mitogen-activated protein kinase (MAPK) signaling has been proposed by our group and others as a key mediator of resistance, but the exact mechanism driving reactivation and the therapeutic implications are unclear. We find that upstream feedback activation of wild-type RAS, as opposed to a shift in KRASG12C to its active guanosine triphosphate (GTP)-bound state, is sufficient to drive RAS-MAPK reactivation in a KRASG12C-independent manner. Moreover, multiple receptor tyrosine kinases (RTKs) can drive feedback reactivation, potentially necessitating targeting of convergent signaling nodes for more universal efficacy. Even in colorectal cancer, where feedback is thought to be primarily epidermal growth factor receptor (EGFR)-mediated, alternative RTKs drive pathway reactivation and limit efficacy, but convergent upstream or downstream signal blockade can enhance activity. Overall, these data provide important mechanistic insight to guide therapeutic strategies targeting KRAS.

Oscillatory amplitudes at 0.1 Hz in both MAP and mean MCAv increased during OLBNP at high altitude. This effect may be due, in part, to the sympathoexcitatory stimulus of hypobaric hypoxia, and does not alter the protection of cerebral tissue oxygenation in this environment.

Introduction Greater low frequency (LF) oscillations (~0.1 Hz) in mean arterial pressure (MAP) and cerebral blood flow (indexed by middle cerebral artery velocity; MCAv) have been associated with higher tolerance to central hypovolemia. As the fast Fourier transform approach was used in these prior studies, measuring exact timing of oscillatory amplitude was not possible. In the present study, the continuous wavelet transform was used for time localization and measurement of the magnitude of LF oscillations during application of lower body negative pressure (LBNP) to presyncope to reduce cerebral perfusion. We hypothesized that the magnitude of LF oscillations in MAP and MCAv would be higher in subjects with greater tolerance to this stress. We also hypothesized that the time of instantaneous maximum magnitude of LF oscillations would be further from baseline for high tolerant subjects. Methods 31 healthy human subjects (14 F: 17 M; 25.1 ± 3.0 y) underwent a stepwise LBNP protocol to presyncope. Subjects were classified as high tolerant (HT) if they completed the -60 mmHg stage of LBNP, and low tolerant (LT) if they did not. Continuous beat-to-beat MAP and mean MCAv were recorded. The continuous wavelet transform was used to extract magnitude of oscillations within the LF range (0.07-0.15 Hz) over time for both MAP and mean MCAv. The instantaneous maximum magnitude of LF oscillations was then extracted. Pearson's correlation was calculated for instantaneous LF magnitude of MAP and mean MCAv versus presyncopal time. A t-test was then used to compare magnitude and relative time of LF oscillations between HT and LT subjects. Results The maximum magnitude of MAP LF oscillations was positively correlated with presyncopal time (R = 0.39, P = 0.03). The maximum magnitude of MCAv LF oscillations, however, was not correlated with presyncopal time (R = -0.03, P = 0.86). When assessing HT and LT subjects, no differences were observed for maximum magnitude of LF oscillations in MAP (HT 4.3 ± 1.0, LT 3.7 ± 1.2, P=0.15) or MCAv (HT 3.1 ± 0.9, LT 3.1 ± 0.7, P=0.91). Time to maximum magnitude of LF oscillations was greater in high tolerant subjects for both MAP (HT 1384.5 ± 329.6 s, LT 798.6 ± 416.8 s, P<0.01) and MCAv (HT 1064.3 ± 431.6 s, LT 662.4 ± 380.1 s, P=0.01). When expressing the time of maximum amplitude as a percent of presyncopal time, the average time in all subjects for MAP was 73.6 ± 22.0 % and 59.6 ± 26.1 % for MCAv, and there were no differences between tolerance groups in either MAP (HT 79.5 ± 14.0 %, LT 67.4 ± 27.3 %, P=0.14) or MCAv (HT 61.8 ± 24.8 %, LT 57.3 ± 28.1 %, P=0.64). Conclusions Using wavelet analysis, only the maximum magnitude of MAP LF oscillations was modestly correlated to presyncopal time. Contrary to previous measures using fast Fourier transform, there was no difference in instantaneous maximum magnitude of oscillations in MAP and mean MCAv between high and low tolerant subjects indicating that both groups could mount similar responses. Interestingly, the relative timing of maximum magnitude of oscillations in MAP and MCAv was no different between tolerance groups.

Sleep is an important determinant of cardiovascular health. Poor sleep quality, including sleep restriction, is associated with impairments in cardiovascular function, and may limit cardiovascular responsiveness to physiological stress. As hemorrhage is one of the leading causes of preventable civilian death, and a major cause of death from trauma in the military, it is important to assess the impact of sleep quality on cardiovascular responses to this stress. We hypothesize that human subjects who report poor sleep quality will be less tolerant to simulated hemorrhage, which will be associated with lower arterial pressure and cerebral blood flow, and higher heart rates compared to subjects who report good sleep quality.

IntroductionHemorrhage (i.e., massive blood loss) induces an oxidative stress and inflammatory response that can persist even following hemostasis and resuscitation. These responses can result in tissue and organ damage if therapeutic inventions are delayed. Pre‐menopausal females exhibit a survival advantage following hemorrhage compared to males of a similar age. The role of oxidative stress and inflammation on increased survival from blood loss in young females is under investigation. Examining the potential mechanisms underpinning this survival advantage following hemorrhage may facilitate the development of sex‐specific therapies. In our laboratory, we utilize lower body negative pressure (LBNP) to simulate blood loss in conscious human subjects. In this study, we hypothesized that young males would elicit a greater oxidative stress and inflammatory response compared to young females, both during and after a simulated hemorrhage via LBNP.MethodsYoung, healthy human subjects (10F; 10M) participated in a stepwise‐LBNP protocol to presyncope. Stroke volume was estimated via finger photoplethysmography as a marker of central hypovolemia (indexed to body surface area). Venous blood samples were collected at baseline, at the onset of presyncope, and 60‐min into recovery (i.e., following “resuscitation”). The oxidative stress response was assessed via measurement of circulating F2‐Isoprostanes (F2‐IsoP) using gas chromatography‐negative ion chemical ionization‐mass spectrometry. The inflammatory response was assessed via measurement of circulating interleukin (IL)‐6 and IL‐10 using a MSD® Multiplex assay. Unpaired t‐tests were used to compare LBNP tolerance and stroke volume responses between male and female subjects. Two factor (time and sex) linear mixed model analyses with repeated measures were performed for all other comparisons, followed by Holm‐corrected post‐hoc tests. All data are represented as mean ± SE.ResultsLBNP tolerance time was similar between male and female subjects (Males, 1592 ± 124 s vs. Females, 1437 ± 113 s; P=0.37), and stroke volume index decreased by a similar magnitude at presyncope (Males, −50.2 ± 6.3% vs. Females, −49.4 ± 3.2%; P = 0.87). There was no effect of time or sex on [F2‐IsoP] or on the %Δ [F2‐IsoP] during or after LBNP (P ≥ 0.12). However, male subjects exhibited a greater increase in both the %Δ [IL‐6] and %Δ [IL‐10] compared to female subjects at the 60‐min recovery time point (IL‐6: Males, 101.4 ± 138.9% vs. Females, 12.3 ± 34.0%; P = 0.06. IL‐10: Males, 71.1 ± 133.3% vs. Females, −2.2 ± 11.8%; P = 0.06).ConclusionThese data suggest that there may be a sex difference in the inflammatory response to blood loss and subsequent fluid resuscitation. Future clinical studies should continue to explore sex differences in inflammatory responses to actual blood loss, and tailor therapeutic interventions accordingly.Support or Funding InformationWilliam and Ella Owens Medical Research Foundation

Increasing resistance to inspiration further decreases intrathoracic and intracranial pressures, leading to increases in venous return, stroke volume, and consequently, arterial pressure and cerebral perfusion pressure. Accordingly, inspiratory resistance breathing may be of therapeutic interest for clinical conditions associated with decreased delivery of blood and oxygen to the brain. Previous studies in healthy humans have investigated the effect of inspiratory resistance breathing on the cerebral circulation, by studying blood velocity within intracranial vessels (via transcranial Doppler (TCD) ultrasound), with equivocal results. The aim of this study was to investigate the effect of inspiratory resistance breathing on blood flow through the internal carotid artery (ICA), and cerebral tissue oxygenation during hypoxia. We hypothesized that inspiratory resistance breathing would increase ICA blood flow, and improve cerebral tissue oxygenation during a mild hypoxic stimulus.MethodsSix healthy human subjects (5M/1F) completed two 10‐min protocols in randomized order: 1) breathing a hypoxic gas mix (16% O2, balance N2), and 2) breathing a hypoxic gas mix (16% O2, balance N2) with inspiratory resistance of −7 cmH2O. ICA blood flow was derived from simultaneous measurements of blood velocity and diameter measured by duplex ultrasound, middle cerebral artery velocity (MCAv) was recorded continuously by TCD ultrasound, and cerebral tissue oxygenation (ScO2) was recorded by near‐infrared spectroscopy. Arterial oxygen saturation (SpO2) was measured by pulse oximetry, and end‐tidal O2 (etO2) was measured by a gas analyzer. Data were analyzed from the last 1‐min of baseline and each protocol with a linear mixed model.ResultsThe hypoxic stimulus decreased SpO2 and etO2 (P<0.0001), and inspiratory resistance breathing did not protect against these responses (P≥0.21). ICA diameter increased with hypoxia under both conditions (P=0.002), regardless of resistance breathing (P=0.56). ICA velocity (23.6 ± 1.6 cm/s vs. 25.7 ± 1.1 cm/s) and flow (298.8 ± 36.6 ml/min vs. 328.8 ± 39.5 ml/min) were lower with hypoxia and inspiratory resistance breathing compared with hypoxia alone (P≤0.07), while mean MCAv was not affected by either hypoxia or resistance breathing (P≥0.15). ScO2 decreased under both conditions (P=0.0009; hypoxia: −2.7 ± 0.9 %; hypoxia + resistance breathing: −3.6 ± 0.8 %), with no effect of inspiratory resistance breathing (P=0.90).ConclusionThese preliminary data demonstrate that, contrary to our hypothesis, inspiratory resistance breathing did not increase ICA blood flow, nor improve cerebral tissue oxygenation during a mild hypoxic stimulus. Rather, ICA blood flow decreased with inspiratory resistance breathing during hypoxia, primarily due to a reduction in ICA blood velocity. Further studies are required to elucidate whether inspiratory resistance breathing may be of therapeutic interest for clinical conditions affecting cerebral blood flow and tissue oxygenation.

IntroductionCerebral tissue oxygenation can be impaired by decreases in oxygen delivery as a result of reduced cerebral blood flow, and environmental conditions such as ascent to high altitude. Recent evidence suggests that an oscillatory pattern in cerebral blood flow (at ~0.1 Hz) may protect cerebral oxygenation under conditions of cerebral hypoperfusion. In this study, we hypothesized that inducing oscillations in cerebral blood flow at 0.1 Hz would protect cerebral blood flow and cerebral tissue oxygen saturation during exposure to combined simulated hemorrhage and sustained hypobaric hypoxia (ascent and partial acclimatization to high altitude).Methods8 healthy human subjects (4 M, 24.7 ± 4.1 y; 4 F, 34.3 ± 8.3 y) participated in two experiments at high altitude (White Mountain, California, USA; altitude, 3800 m): 1) a control condition (CTRL) where lower body negative pressure (LBNP) was used to induce central hypovolemia by reducing chamber pressure to −60 mmHg for 10‐min, and 2) oscillatory LBNP (OLBNP) where chamber pressure was reduced to −60 mmHg, then oscillated every 5‐s between −30 mmHg and −90 mmHg for 10‐min (0.1 Hz). Measurements included internal carotid artery (ICA) blood flow via duplex Doppler ultrasound, middle cerebral artery velocity (MCAv) via transcranial Doppler ultrasound, and cerebral tissue oxygen saturation via near‐infrared spectroscopy. Frequency analysis (via fast Fourier transform) was performed to verify that oscillations in mean MCAv were generated at ~0.1 Hz. Data were analyzed with a linear mixed‐model. All data are represented as mean ± SE.ResultsLow frequency power (0.07–0.15 Hz) in mean MCAv increased during OLBNP vs. CTRL (P = 0.02). OLBNP did not protect ICA flow (OLBNP: −32.5 ± 4.5 Δ%; CTRL: −19.9 ± 8.9 Δ%; P = 0.18) or mean MCAv (OLBNP: −18.5 ± 3.4 Δ%; CTRL: −15.3 ± 5.4 Δ%; P = 0.58), but cerebral tissue oxygenation was protected (OLBNP: −0.67 ± 1.0 Δ%; CTRL: −4.07 ± 2.0 Δ%; P = 0.004).ConclusionsThese results support our hypothesis that inducing oscillatory blood flow leads to protection of cerebral tissue oxygenation, despite no differences in ICA blood flow or mean MCAv. Overall, these data suggest that therapies using oscillatory perfusion may help preserve cerebral tissue oxygen saturation under conditions of reduced oxygen delivery.Support or Funding InformationAHA 17GRNT33671110

IntroductionTrauma‐induced hemorrhage can occur at high altitude from a variety of causes, including battlefield injuries, motor vehicle accidents, air accidents, and major falls. The hypoxic environment of high altitude can limit the ability of the cardiovascular system to compensate for blood loss injuries, due, in part, to the reduced arterial oxygen content. In humans, the effect of sustained hypoxia on tolerance to hemorrhage and the cardiovascular and cerebrovascular responses to this stress are unknown. Based on the known compensatory increases in cerebral blood flow that occur with exposure to hypoxia, we hypothesized that tolerance to simulated hemorrhage (via application of lower body negative pressure, LBNP) at high altitude would be similar compared to low altitude due to increased cerebral blood flow and oxygen delivery, and the subsequent preservation of cerebral tissue oxygenation.MethodsHealthy human subjects (N=8; 4F, 4M) participated in LBNP protocols to presyncope at low altitude (1045 m, Calgary, Canada) and at high altitude (3800 m, White Mountain, California) following 4–5 days of acclimatization. LBNP chamber pressure was initially reduced to −60 mmHg for 10‐min followed by decreases every 5‐min to −70, −80, −90 and −100 mmHg, until the onset of presyncopal symptoms. Arterial pressure, heart rate, internal carotid artery blood flow, and cerebral oxygen saturation were measured continuously. Stroke volume was derived from the arterial pressure waveform, and systemic vascular resistance was calculated from cardiac output and mean arterial pressure. Time to presyncope and cardiovascular responses were compared between the low and high altitude conditions.ResultsTime to presyncope was similar between conditions (low altitude: 1276 ± 108 s vs. high altitude: 1208± 108 s; P=0.58). Similar responses to LBNP were observed between low and high altitudes in mean arterial pressure (low altitude: −16±2 % vs. high altitude: −16±2 %; P=0.85), stroke volume (low altitude: −57±5 % vs. high altitude: −60±5 %; P=0.39), systemic vascular resistance (low altitude: +23±9 % vs. high altitude: +38±12 %; P=0.21), and heart rate (low altitude: +69±12 % vs. high altitude: +65±8 %; P=0.71). Internal carotid artery blood flow was higher at high altitude vs. low altitude (condition effect, P=0.01), and decreased with LBNP under both conditions (P≤0.005). There was no effect of high altitude on cerebral oxygen saturation at baseline and presyncope (altitude Effect, P=0.73).ConclusionThese findings suggest that the hypoxia induced by ascent to high altitude (3800 m) does not affect tolerance to simulated hemorrhage in young healthy adults, which may be due to 1) similar cardiovascular reflex responses to central hypovolemia, and/or 2) the compensatory increase in cerebral blood flow and subsequent maintenance of oxygen delivery to the tissues, resulting in the preservation of cerebral oxygen saturation.Support or Funding InformationAHA 17GRNT33671110