Attempted replication of pharmacogenetic association of variants in PPP1R14C and CCDC148 with aromatase inhibitor-induced musculoskeletal symptoms

Third-generation aromatase inhibitors (AI) are the standard treatment for patients with hormone receptor positive (HR+) breast cancer. While effective, AI can lead to severe adverse events, including AI-induced musculoskeletal syndrome (AIMSS). Genetic predictors of AIMSS have the potential to personalize AI treatment and improve outcomes. We attempted to replicate results from a previous genome-wide association study that found a lower risk of AIMSS in patients carrying PPP1R14C rs912571 and a higher risk in patients carrying CCDC148 rs79048288. AIMSS data were collected prospectively from patients with HR+ breast cancer prior to starting and after 3 and 6 months of adjuvant AI via the Patient-Reported Outcome Measurement Information System and Functional Assessment of Cancer Therapy-Endocrine Symptom. Germline genotypes for PPP1R14C rs912571 and CCDC148 rs79048288 were tested for a similar association with AIMSS as previously reported via $2 tests. Of the 143 patients with AIMSS and genetics data were included in the analysis. There was no association identified between PPP1R14C rs912571 and AIMSS risk ( P  > 0.05). Patients carrying CCDC148 rs79048288 variant alleles had lower AIMSS incidence in a secondary analysis ( P  = 0.04); however, this was in the opposite direction of the previous finding. The study did not replicate previously reported associations with AIMSS risk for genetic variants in PPP1R14C and CCDC148 and AIMSS risk. Further research is needed to discover and validate genetic predictors of AIMSS that can be used to personalize treatment in patients with HR+ breast cancer.

Discovery of as a Potent and Orally Efficacious Estrogen Receptor PROTAC Degrader with Strong Antitumor Activity

Estrogen receptor α (ERα) is a prime target for the treatment of ER-positive (ER+) breast cancer. Despite the development of several effective therapies targeting ERα signaling, clinical resistance remains a major challenge. In this study, we report the discovery of a new class of potent and orally bioavailable ERα degraders using the PROTAC technology, with ERD-3111 being the most promising compound. ERD-3111 exhibits potent in vitro degradation activity against ERα and demonstrates high oral bioavailability in mice, rats, and dogs. Oral administration of ERD-3111 effectively reduces the levels of wild-type and mutated ERα proteins in tumor tissues. ERD-3111 achieves tumor regression or complete tumor growth inhibition in the parental MCF-7 xenograft model with wild-type ER and two clinically relevant ESR1 mutated models in mice. ERD-3111 is a promising ERα degrader for further extensive evaluations for the treatment of ER+ breast cancer.

Effects of CYP3A4 and CYP2C9 genotype on systemic anastrozole and fulvestrant concentrations in SWOG S0226

Objective & methods: This study tested associations of genotype-predicted activity of CYP3A4, other pharmacogenes, SLC28A7 (rs11648166) and ALPPL2 (rs28845026) with systemic concentrations of the endocrine therapies anastrozole and fulvestrant in SWOG S0226 trial participants. Results: Participants in the anastrozole-only arm with low CYP3A4 activity (i.e. CYP3A4*22 carriers) had higher systemic anastrozole concentrations than patients with high CYP3A4 activity (β-coefficient = 10.03; 95% CI: 1.42, 18.6; p = 0.025). In an exploratory analysis, participants with low CYP2C9 activity had lower anastrozole concentrations and higher fulvestrant concentrations than participants with high CYP2C9 activity. Conclusion: Inherited genetic variation in CYP3A4 and CYP2C9 may affect concentrations of endocrine therapy and may be useful to personalize dosing and improve treatment outcomes.

Genome-wide association study of aromatase inhibitor discontinuation due to musculoskeletal symptoms

OBJECTIVE

Aromatase inhibitors (AIs) are commonly used to treat hormone receptor positive (HR +) breast cancer. AI-induced musculoskeletal syndrome (AIMSS) is a common toxicity that causes AI treatment discontinuation. The objective of this genome-wide association study (GWAS) was to identify genetic variants associated with discontinuation of AI therapy due to AIMSS and attempt to replicate previously reported associations.

METHODS

In the Exemestane and Letrozole Pharmacogenetics (ELPh) study, postmenopausal patients with HR + non-metastatic breast cancer were randomized to letrozole or exemestane. Genome-wide genotyping of germline DNA was conducted followed by imputation. Each imputed variant was tested for association with time-to-treatment discontinuation due to AIMSS using a Cox proportional hazards model assuming additive genetic effects and adjusting for age, baseline pain score, prior taxane treatment, and AI arm. Secondary analyses were conducted within each AI arm and analyses of candidate variants previously reported to be associated with AIMSS risk.

RESULTS

Four hundred ELPh participants were included in the combined analysis. Two variants surpassed the genome-wide significance level in the primary analysis (p value < 5 × 10-8), an intronic variant (rs79048288) within CCDC148 (HR = 4.42, 95% CI: 2.67-7.33) and an intergenic variant (rs912571) upstream of PPP1R14C (HR = 0.30, 95% CI: 0.20-0.47). In the secondary analysis, rs74418677, which is known to be associated with expression of SUPT20H, was significantly associated with discontinuation of letrozole therapy due to AIMSS (HR = 5.91, 95% CI: 3.16-11.06). We were able to replicate associations for candidate variants previously reported to be associated with AIMSS in this cohort, but were not able to replicate associations for any other variants previously reported in other patient cohorts.

CONCLUSIONS

Our GWAS findings identify several candidate variants that may be associated with AIMSS risk from AI generally or letrozole specifically. Validation of these associations in independent cohorts is needed before translating these findings into clinical practice to improve treatment outcomes in patients with HR + breast cancer.

PD-L1 expression on circulating tumor cells and platelets in patients with metastatic breast cancer

Background

Immune checkpoint inhibition is effective in several cancers. Expression of programmed death-ligand 1 (PD-L1) on circulating tumor or immune effector cells could provide insights into selection of patients for immune checkpoint inhibition.

Methods

Whole blood was collected at serial timepoints from metastatic breast cancer patients and healthy donors for circulating tumor cell (CTC) and platelet PD-L1 analysis with a phycoerythrin-labeled anti-human PD-L1 monoclonal antibody (Biolegend clone 29E.2A3) using the CellSearch^® assay. CTC PD-L1 was considered positive if detected on at least 1% of the cells; platelet PD-L1 was considered positive if ≥100 platelets per CellSearch frame expressed PD-L1.

Results

A total of 207 specimens from 124 metastatic breast cancer patients were collected. 52/124 (42%) samples at timepoint-1 (at or close to time of progressive disease) had ≥5 CTC/7.5ml whole blood. Of those, 21 (40%) had positive CTC PD-L1. In addition, platelet PD-L1 expression was observed in 35/124 (28%) at timepoint-1. Platelet PD-L1 was not detected in more than 70 specimens from 12 healthy donors. Platelet PD-L1 was associated with ≥5 CTC/7.5ml whole blood (p = 0.0002), less likely in patients with higher red blood cell counts (OR = 0.72, p<0.001) and a history of smoking tobacco (OR = 0.76, p<0.001). Platelet PD-L1 staining was not associated with tumor marker status, recent procedures or treatments, platelet-affecting drugs, or CTC PD-L1 expression.

Conclusion

PD-L1 expression was found in metastatic breast cancer patients on both CTC and platelets in an independent fashion. Inter-patient platelet PD-L1 expression was highly heterogeneous suggesting that it is a biological event associated with cancer in some but not all patients. Taken together, our data suggest that CTC and platelet PD-L1 expression could play a role in predicting which patients should receive immune checkpoint inhibition and as a pharmacodynamics biomarker during treatment.

Impact of Pharmacogenetics on Intravenous Tacrolimus Exposure and Conversions to Oral Therapy

CYP3A5 and CYP3A4 are the predominant enzymes responsible for tacrolimus metabolism; however only a proportion of the population expresses CYP3A5 secondary to genetic variation. CYP3A5 is expressed in both the intestine and the liver and has been shown to impact both the bioavailability and metabolism of orally administered tacrolimus. Increasing the initial tacrolimus dose by 50% to 100% is recommended in patients who are known CYP3A5 expressers; however, whether this dose adjustment is appropriate for i.v. tacrolimus administration is unclear. The objective of this study was to evaluate the impact of CYP3A5 genotype as well as other pharmacogenes on i.v. tacrolimus exposure to determine whether the current genotype-guided dosing recommendations are appropriate for this formulation. In addition, this study aimed to investigate dose conversion requirements among CYP3A5 genotypes when converting from i.v. to p.o. tacrolimus. This study is a retrospective chart review of all patients who underwent allogeneic stem cell transplantation at Michigan Medicine between June 1, 2014, and March 1, 2018, who received i.v. tacrolimus at the time of their transplantation. Secondary use samples were obtained for genotyping CYP3A5, CYP3A4, and ABCB1. Patient demographic information, tacrolimus dosing and trough levels, and concomitant medications received at the time of tacrolimus trough were collected retrospectively from the patients' medical records. The i.v. dose-controlled concentration (C/D) and the i.v.:p.o. exposure ratio was calculated for all tacrolimus doses and patients, respectively. The impact of CYP3A5, CYP3A4, and ABCB1 genotypes on the i.v. C/D were evaluated with linear mixed modeling. The impact of CYP3A5 genotype on the i.v.:p.o. ratio was evaluated while controlling for age and concomitant use of an azole inhibitor. CYP3A5 and CYP3A4 genotypes were significantly associated with the i.v. C/D, with CYP3A5 expressers and CYP3A4 rapid metabolizers having 20% lower tacrolimus exposure. Neither genotype remained significant in the multivariable model, although age, hematocrit, and concomitant use of strong azole inhibitors were associated with increased i.v. C/D. When controlling for patient age and sex, CYP3A5 expressers had significantly higher i.v.:p.o. ratios than CYP3A5 nonexpressers (3.42 versus 2.78; P = .04). Post hoc analysis showed that the i.v.:p.o. ratio may differ among different CYP3A5 genotypes and azole inhibitor combinations. This study demonstrates that the current genotype-guided tacrolimus dose adjustment recommendations are inappropriate for CYP3A5 expressers receiving i.v. tacrolimus. Although CYP3A5 genotype is likely a minor contributor to i.v. tacrolimus exposure, genotype, in addition to capturing concomitant CYP3A inhibitors, would likely improve i.v.:p.o. dose conversion selection. © 2021 American Society for Transplantation and Cellular Therapy. Published by Elsevier Inc.

Genome-wide association study of letrozole plasma concentrations identifies non-exonic variants that may affect CYP2A6 metabolic activity

OBJECTIVES

Letrozole is a nonsteroidal aromatase inhibitor used to treat hormone-receptor-positive breast cancer. Variability in letrozole efficacy and toxicity may be partially attributable to variable systemic drug exposure, which may be influenced by germline variants in the enzymes responsible for letrozole metabolism, including cytochrome P450 2A6 (CYP2A6). The objective of this genome-wide association study (GWAS) was to identify polymorphisms associated with steady-state letrozole concentrations.

METHODS

The Exemestane and Letrozole Pharmacogenetics (ELPh) Study randomized postmenopausal patients with hormone-receptor-positive nonmetastatic breast cancer to letrozole or exemestane treatment. Germline DNA was collected pretreatment and blood samples were collected after 1 or 3 months of treatment to measure steady-state letrozole (and exemestane) plasma concentrations via HPLC/MS. Genome-wide genotyping was conducted on the Infinium Global Screening Array (>650 000 variants) followed by imputation. The association of each germline variant with age- and BMI-adjusted letrozole concentrations was tested in self-reported white patients via linear regression assuming an additive genetic model.

RESULTS

There were 228 patients who met the study-specific inclusion criteria and had both DNA and letrozole concentration data for this GWAS. The association for one genotyped polymorphism (rs7937) with letrozole concentration surpassed genome-wide significance (P = 5.26 × 10-10), explaining 13% of the variability in untransformed steady-state letrozole concentrations. Imputation around rs7937 and in silico analyses identified rs56113850, a variant in the CYP2A6 intron that may affect CYP2A6 expression and activity. rs7937 was associated with age- and BMI-adjusted letrozole levels even after adjusting for genotype-predicted CYP2A6 metabolic phenotype (P = 3.86 × 10-10).

CONCLUSION

Our GWAS findings confirm that steady-state letrozole plasma concentrations are partially determined by germline polymorphisms that affect CYP2A6 activity, including variants near rs7937 such as the intronic rs56113850 variant. Further research is needed to confirm whether rs56113850 directly affects CYP2A6 activity and to integrate nonexonic variants into CYP2A6 phenotypic activity prediction systems.

Further Evidence That OPG rs2073618 Is Associated With Increased Risk of Musculoskeletal Symptoms in Patients Receiving Aromatase Inhibitors for Early Breast Cancer

Background

Aromatase inhibitors (AI) reduce recurrence and death in patients with early-stage hormone receptor-positive (HR +) breast cancer. Treatment-related toxicities, including AI-induced musculoskeletal symptoms (AIMSS), are common and may lead to early AI discontinuation. The objective of this study was to replicate previously reported associations for candidate germline genetic polymorphisms with AIMSS.

Methods

Women with stage 0-III HR + breast cancer initiating adjuvant AI were enrolled in a prospective clinic-based observational cohort. AIMSS were assessed by patient-reported outcomes (PRO) including the PROMIS pain interference and physical function measures plus the FACT-ES joint pain question at baseline and after 3 and 6 months. For the primary analysis, AIMSS were defined as ≥ 4-point increase in the pain interference T-score from baseline. Secondary AIMSS endpoints were defined as ≥ 4-point decrease in the physical function T-score from baseline and as ≥ 1-point increase on the FACT-ES joint pain question from baseline. The primary hypothesis was that TCL1A rs11849538 would be associated with AIMSS. Twelve other germline variants in CYP19A1, VDR, PIRC66, OPG, ESR1, CYP27B1, CYP17A1, and RANKL were also analyzed assuming a dominant genetic effect and prespecified direction of effect on AIMSS using univariate logistic regression with an unadjusted α = 0.05. Significant univariate associations in the expected direction were adjusted for age, race, body mass index (BMI), prior taxane, and the type of AI using multivariable logistic regression.

Results

A total of 143 participants with PRO and genetic data were included in this analysis, most of whom were treated with anastrozole (78%) or letrozole (20%). On primary analysis, participants carrying TCL1A rs11849538 were not more likely to develop AIMSS (odds ratio = 1.29, 95% confidence interval: 0.55-3.07, p = 0.56). In the statistically uncorrected secondary analysis, OPG rs2073618 was associated with AIMSS defined by worsening on the FACT-ES joint pain question (OR = 3.33, p = 0.004), and this association maintained significance after covariate adjustment (OR = 3.98, p = 0.003).

Conclusion

Carriers of OPG rs2073618 may be at increased risk of AIMSS. If confirmed in other cohorts, OPG genotyping can be used to identify individuals with HR + early breast cancer in whom alternate endocrine therapy or interventions to enhance symptom detection and implement strategies to reduce musculoskeletal symptoms may be needed.

Evaluating the Impact of CYP3A5 Genotype on Post-Transplant Healthcare Resource Utilization in Pediatric Renal and Heart Transplant Recipients Receiving Tacrolimus

Purpose

CYP3A5 genotype is a significant contributor to inter-individual tacrolimus exposure and may impact the time required to achieve therapeutic concentrations and number of tacrolimus dose adjustments in transplant patients. Increased modifications to tacrolimus therapy may indicate a higher burden on healthcare resources. The purpose of this study was to evaluate whether CYP3A5 genotype was predictive of healthcare resource utilization in pediatric renal and heart transplant recipients.

Patients and Methods

Patients <18 years of age with a renal or heart transplant between 6/1/2014–12/31/2018 and tacrolimus-based immunosuppression were included. Secondary use samples were obtained for CYP3A5 genotyping. Clinical data was retrospectively collected from the electronic medical record. Healthcare resource utilization measures included the number of dose changes, number of tacrolimus concentrations, length of stay, number of clinical encounters, and total charges within the first year post-transplant. Rejection and donor-specific antibody (DSA) formation within the first year were also collected. The impact of CYP3A5 genotype was evaluated via univariate analysis for the first year and multivariable analysis at 30, 90, 180, 270, and 365 days post-transplant.

Results

Eighty-five subjects were included, 48 renal transplant recipients and 37 heart transplant recipients. CYP3A5 genotype was not associated with any outcomes in renal transplant, however, a CYP3A5 expresser phenotype was a predictor of more dose changes, more tacrolimus concentrations, longer length of stay, and higher total charges in heart transplant recipients. CYP3A5 genotype was not associated with rejection or DSA formation. Age and induction therapy were associated with higher total charges.

Conclusion

CYP3A5 genotype may predict healthcare resource utilization in the first year post-transplant, although this may be mitigated by differences in tacrolimus management. Future studies should evaluate the impact of genotype-guided dosing strategies for tacrolimus on healthcare utilization resources.

Abstract PS5-25: Association of OPG rs2073618 and aromatase inhibitor induced musculoskeletal symptoms

Background: Adjuvant aromatase inhibitors (AI) reduce recurrence and mortality after early stage hormone receptor-positive (HR+) breast cancer (BC). Treatment-related toxicities, including AI-induced musculoskeletal symptoms (AIMSS), are common during adjuvant AI therapy. Prior work suggests the risk of AIMSS is associated with inherited germline genetic polymorphisms in several genes, such as TCL1A, CYP19A1, OPG, and VDR. These pharmacogenetic associations require replication in independent cohorts prior to clinical translation to identify patients at risk for AIMSS. The objective of this retrospective pharmacogenetic analysis was to replicate previously reported associations for candidate germline genetic polymorphisms with AIMSS. Methods: Women with stage 0-III HR+ BC initiating adjuvant endocrine therapy (ET) with tamoxifen or an AI were enrolled in a prospective clinic-based observational cohort. The type of ET was selected by the provider. A baseline (BL) blood sample was collected for isolation of germline DNA for pharmacogenetic analysis. AIMSS were assessed by patient-reported outcomes (PRO). Participants completed PROMIS pain interference (PI), PROMIS physical function (PF) and Functional Assessment of Cancer Therapy - Endocrine Symptoms (FACT-ES) measures at BL and after 3, 6, 9, 12, 24, 48 and 60 months (mo). The FACT-ES includes one question about joint pain, rated on a 5-point scale (“not at all” to “very much”). This secondary retrospective pharmacogenetic analysis was conducted in participants receiving AI therapy for whom blood samples and PRO scores at BL and 3 and/or 6 mo were available. For the primary analysis, we defined AIMSS as a ≥4 point increase in PI T score from BL to 3 and/or 6 mo. For secondary analyses, we evaluated alternate definitions of AIMSS including a ≥4 point decrease in PF T score and a ≥1 category increase on the FACT-ES joint pain question from BL to 3 and/or 6 mo. The primary hypothesis was that TCL1A rs11849538 is associated with AIMSS. Twelve other germline variants in CYP19A1, VDR, PIRC66, OPG, ESR1, CYP27B1, CYP17A1, and RANKL previously reported to be associated with AIMSS were also analyzed. We assumed a dominant genetic effect and pre-specified the direction of effect on AIMSS for each variant. We conducted univariate logistic regression to evaluate associations between each definition of AIMSS and candidate polymorphism using an unadjusted α=0.05. Significant univariate associations in the expected direction were adjusted for age, race, body mass index (BMI), prior taxane and type of AI using multivariable logistic regression. Results: Of 182 participants on AI, 143 with PRO and genetic data available were included in this analysis. Median age was 67, 85% were white, median BMI was 27.8 and 27% had prior taxane. 78% received anastrozole, 20% letrozole and 2% exemestane. On primary analysis, participants carrying TCL1A rs11849538 were not more likely to develop AIMSS as defined by increase in PI T score by ≥4 (OR=1.29, 95% CI: 0.55-3.07, p=0.56). None of the other polymorphisms was associated with increase in PI T score by ≥4. On secondary analysis, OPG rs2073618 was associated with AIMSS, as defined by an increase on the FACT-ES joint pain question ≥1 (OR=3.33, 95% CI: 1.48-7.49, p=0.004) and this association maintained significance after covariate adjustment (OR=3.98, 95% CI: 1.61-9.84, p=0.003). Age, race, BMI, prior taxane and type of AI were not associated with AIMSS on multivariate analysis. No other polymorphisms were associated with AIMSS on secondary analyses.

Conclusions: Carriers of OPG rs2073618 may be at increased risk of AIMSS. If confirmed in other cohorts, OPG genotyping may be able to identify individuals with HR+ early BC at increased risk for AIMSS during AI therapy. Alternate ET or interventions to reduce musculoskeletal symptoms may be needed for this population.

Citation Format: Daniel L Hertz, Karen Lisa Smith, Yuhua Zong, Christina L Gersch, Andrea Pesch, Arti Patel, Jennifer Lehman, N. Lynn Henry, Kelley M Kidwell, James M Rae, Vered Stearns. Association of OPG rs2073618 and aromatase inhibitor induced musculoskeletal symptoms [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-25.

Short-term CDK4/6 Inhibition Radiosensitizes Estrogen Receptor-Positive Breast Cancers

PURPOSE

Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors have improved progression-free survival for metastatic, estrogen receptor-positive (ER⁺) breast cancers, but their role in the nonmetastatic setting remains unclear. We sought to understand the effects of CDK4/6 inhibition (CDK4/6i) and radiotherapy in multiple preclinical breast cancer models.

EXPERIMENTAL DESIGN

Transcriptomic and proteomic analyses were used to identify significantly altered pathways after CDK4/6i. Clonogenic assays were used to quantify the radiotherapy enhancement ratio (rER). DNA damage was quantified using γH2AX staining and the neutral comet assay. DNA repair was assessed using RAD51 foci formation and nonhomologous end joining (NHEJ) reporter assays. Orthotopic xenografts were used to assess the efficacy of combination therapy.

RESULTS

Palbociclib significantly radiosensitized multiple ER⁺ cell lines at low nanomolar, sub IC₅₀ concentrations (rER: 1.21-1.52) and led to a decrease in the surviving fraction of cells at 2 Gy (P < 0.001). Similar results were observed in ribociclib-treated (rER: 1.08-1.68) and abemaciclib-treated (rER: 1.19-2.05) cells. Combination treatment decreased RAD51 foci formation (P < 0.001), leading to a suppression of homologous recombination activity, but did not affect NHEJ efficiency (P > 0.05). Immortalized breast epithelial cells and cells with acquired resistance to CDK4/6i did not demonstrate radiosensitization (rER: 0.94-1.11) or changes in RAD51 foci. In xenograft models, concurrent palbociclib and radiotherapy led to a significant decrease in tumor growth.

CONCLUSIONS

These studies provide preclinical rationale to test CDK4/6i and radiotherapy in women with locally advanced ER⁺ breast cancer at high risk for locoregional recurrence.

Abstract 6280: CDK4/6 inhibitor-mediatated radiosensitization of estrogen receptor positive breast cancer

Purpose: Although cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors like palbociclib, ribociclib, and abemaciclib have improved progression free survival in patients with metastatic, estrogen receptor positive (ER+) breast cancer, acquired resistance to these drugs limits their efficacy. Despite promising new studies defining the utility of CDK4/6 inhibitors in the upfront, non-metastatic setting, there is limited data available on the effects of concurrent CDK4/6 inhibition and radiation (RT).

Methods: Transcriptomic and proteomic expression data was used to quantify changes in RNA and protein expression in ER+ breast cancer cell lines (MCF-7, T47D) after short term (16 hour) CDK4/6 inhibition. Proliferation assays were used to determine the half-maximal inhibitory concentration (IC50) of palbociclib, ribociclib, and abemaciclib. Clonogenic survival assays were performed to calculate the radiation enhancement ratio (rER) and the surviving fraction at 2 Gy for each treatment. Homologous recombination (HR) proficiency was assessed using RAD51 and γH2AX foci formation and a pYFP reporter was used to assess non-homologous end joining (NHEJ) efficiency. Western blots were used to quantify protein expression. MCF-7 xenografts were used to study the efficacy of combination (palbociclib + RT) therapy in vivo. MCF-7 and T47D cell lines with acquired resistance to CDK4/6 inhibition (IC50 &gt;1uM) were used for comparison in all assays.

Results: Transcriptomic and proteomic analyses identified changes in expression of DNA damage response mediators and cell cycle machinery with short term CDK4/6 inhibition. Palbociclib significantly radiosensitized ER+ cell lines at concentrations at or below the IC50 value in clonogenic survival assays (MCF-7 rER: 1.22-1.52, T47D rER: 1.23-1.50) and led to a decrease in the surviving fraction of cells at 2 Gy (p &lt; 0.001). Similar results were observed in ribociclib- (rER: 1.08 - 1.68) and abemaciclib-treated (rER: 1.19 - 2.05) cells. MCF-7 and T47D cells treated with CDK4/6 inhibition and RT showed a decrease in RAD51 foci formation, suggesting a decrease in HR efficiency (p &lt; 0.001). However, CDK4/6 inhibition did not affect NHEJ efficiency (p &gt; 0.05). CDK4/6 inhibition + RT led to a decrease in expression of protein expression of HR meditators like p-CHK1 but did not affect phosphorylation of NHEJ proteins like pKu80/pKu70. Cells with acquired resistance to CDK4/6 inhibition did not demonstrate radiosensitization (MCF-7 rER: 0.93 - 1.03, T47D rER: 0.96 - 1.11) or changes in RAD51 foci formation with combination treatment.

Conclusions: Our data suggests that CDK4/6 inhibitor-mediated radiosensitization may be effective in ER+ breast cancers prior to the development of CDK4/6 inhibitor resistance. These studies provide preclinical rationale to test CDK4/6 inhibition + RT in women with locally-advanced ER+ breast cancer at high risk for locoregional recurrence.

Citation Format: Andrea M. Pesch, Nicole Hirsh, Benjamin C. Chandler, Anna R. Michmerhuizen, Cassandra L. Ritter, Marlie Androsiglio, Kari Wilder-Romans, Meilan Liu, Christina L. Gersch, Jose M. Larios, James M. Rae, Corey W. Speers. CDK4/6 inhibitor-mediatated radiosensitization of estrogen receptor positive breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6280.

Genetic variation in Charcot-Marie-Tooth genes contributes to sensitivity to paclitaxel-induced peripheral neuropathy

Aim: This study explored whether inherited variants in genes causing the hereditary neuropathy condition Charcot-Marie-Tooth disease are associated with sensitivity to paclitaxel-induced peripheral neuropathy (PN). Patients & methods: Hereditary neuropathy genes previously associated with risk of paclitaxel-induced PN were sequenced in paclitaxel-treated patients. Eight putative genetic predictors in five hereditary neuropathy genes (ARHGEF10, SBF2, FGD4, FZD3 and NXN) were tested for association with PN sensitivity after accounting for systemic exposure and clinical variables. Results: FZD3 rs7833751, a proxy for rs7001034, decreased PN sensitivity (additive model, β = -0.41; 95% CI: -0.66 to -0.17; p = 0.0011). None of the other genetic predictors were associated with PN sensitivity. Conclusion: Our results support prior evidence that FZD3 rs7001034 is protective of PN and may be useful for individualizing paclitaxel treatment to prevent PN.

Genome-wide association study of steady-state letrozole concentration in patients with breast cancer

538

Background: Letrozole is a non-steroidal aromatase inhibitor (AI) used to treat hormone receptor positive (HR+) breast cancer. Variability in letrozole efficacy and toxicity may be partially attributable to variable systemic drug exposure, which may be influenced by germline variants in the enzymes responsible for letrozole metabolism, including CYP2A6. The objective of this genome-wide association study (GWAS) was to identify genetic variants that affect steady state letrozole concentrations. Methods: The Exemestane and Letrozole Pharmacogenetics (ELPh) Study randomized 503 post-menopausal patients with HR+ non-metastatic breast cancer to exemestane or letrozole treatment. Germline DNA was collected pre-treatment and blood samples were collected after 1 or 3 months of treatment to measure steady-state letrozole concentration via HPLC/MS. Genome-wide genotyping was conducted on the Infinium Global Screening Array to the Haplotype Reference Panel ( > 2 million variants). The association of each polymorphism with square-root transformed letrozole concentration was tested in self-reported white patients via linear regression using the standard alpha for genome-wide significance (α = 5x10−8) assuming an additive genetic model and correcting for age and body mass index. Results: 228 patients met inclusion criteria and had all necessary data. Each variant allele of rs7937 a patient carried increased their letrozole concentration ~22.9 ng/mL (standard error = 4.01, p = 3.51x10−8, Table) and this variant explained 13% of the variability in letrozole concentrations. rs7937 is located ~50 kB upstream of CYP2A6, and has previously been identified in GWAS of CYP2A6-related phenotypes, including nicotine metabolism and lung cancer. Conclusions: This GWAS confirmed that steady-state letrozole concentrations are partially determined by germline polymorphisms affecting CYP2A6 activity. If letrozole concentrations affect treatment efficacy or toxicity, CYP2A6 genetics may be useful to individualize letrozole dosing to improve clinical outcomes in patients with HR+ breast cancer. [Table: see text]

Effects of SLCO1B1 polymorphisms on plasma estrogen concentrations in women with breast cancer receiving aromatase inhibitors exemestane and letrozole

Aim: This study tested for associations between SLCO1B1 polymorphisms and circulating estrogen levels in women with breast cancer treated with letrozole or exemestane. Patients & methods: Postmenopausal women with hormone-receptor positive breast cancer were genotyped for SLCO1B1*5 (rs4149056) and rs10841753. Pretreatment and on-treatment plasma estrogens and aromatase inhibitor (AI) concentrations were measured. Regression analyses were performed to test for pharmacogenetic associations with estrogens and drug concentrations. Results: SLCO1B1*5 was associated with elevated pretreatment estrone sulfate and an increased risk of detectable estrone concentrations after 3 months of AI treatment. Conclusion: These findings suggest SLCO1B1 polymorphisms may have an effect on estrogenic response to AI treatment, and therefore may adversely impact the anticancer effectiveness of these agents.

Correction to: Targeted degradation of activating estrogen receptor α ligand-binding domain mutations in human breast cancer

In the original publication of the article, the spelling of the sixth author's given name was incorrect. The corrected author name should read as "Wadie David". The original article has been corrected.

Targeted degradation of activating estrogen receptor α ligand-binding domain mutations in human breast cancer

PURPOSE

Studies have identified several estrogen receptor α (ERα) ligand-binding domain (LBD) somatic mutations in endocrine therapy resistant, metastatic ER-positive breast cancers. The most common mutations, Tyr537Ser (Y537S) and Asp538Gly (D538G), are detected in ~ 30% of endocrine resistant metastatic breast cancer patients. These ESR1 mutations induce the agonist conformation of ERα, confer an estrogen-independent phenotype, and promote drug resistance to antiestrogens.

METHODS

ER-positive, estrogen-dependent MCF-7 cells were engineered to express either the Y537S or D538G mutants using CRISPR knock-in (cY537S and cD538G). These cells were used to screen several estrogen receptor degrader (ERD) compounds synthesized using the Proteolysis Targeting Chimeras (PROTAC) method to induce degradation of ERα via the ubiquitin-proteasome pathway.

RESULTS

Wild-type MCF-7 and ERα LBD mutant cells were treated with ERD-148 (10 pM-1 µM) and assayed for cellular proliferation using the PrestoBlue cell viability assay. ERD-148 attenuated ER-dependent growth with IC₅₀ values of 0.8, 10.5, and 6.1 nM in MCF-7, cY537S, and cD538G cells, respectively. Western blot analysis showed that MCF-7 cells treated with 1 nM ERD-148 for 24 h exhibited reduced ERα protein expression as compared to the mutants. The ER-regulated gene, GREB1, demonstrated significant downregulation in parental and mutant cells after 24 h of ERD-148 treatment at 10 nM. Growth of the ER-negative, estrogen-independent MDA-MB-231 breast cancer cells was not inhibited by ERD-148 at the ~ IC₉₀ observed in the ER-positive cells.

CONCLUSION

ERD-148 inhibits the growth of ER-positive breast cancer cells via downregulating ERα with comparable potency to Fulvestrant with marginal non-specific toxicity.

Genetic variation in EPHA contributes to sensitivity to paclitaxel-induced peripheral neuropathy

AIMS

Chemotherapy-induced peripheral neuropathy (PN) is a treatment limiting toxicity of paclitaxel. We evaluated if EPHA genetic variation (EPHA4, EPHA5, EPHA6, and EPHA8) is associated with PN sensitivity by accounting for variability in systemic paclitaxel exposure (time above threshold).

METHODS

Germline DNA from 60 patients with breast cancer was sequenced. PN was measured using the 8-item sensory subscale (CIPN8) of the patient-reported CIPN20. Associations for 3 genetic models were tested by incorporating genetics into previously published PN prediction models integrating measured paclitaxel exposure and cumulative treatment. Significant associations were then tested for association with PN-related treatment disruption.

RESULTS

EPHA5 rs7349683 (minor allele frequency = 0.32) was associated with increased PN sensitivity (β-coefficient = 0.39, 95% confidence interval 0.11-0.67, p = 0.007). Setting a maximum tolerable threshold of CIPN8 = 30, optimal paclitaxel exposure target is shorter for rs7349683 homozygous (11.6 h) than heterozygous (12.6 h) or wild-type (13.6 h) patients. Total number of missense variants (median = 0, range 0-2) was associated with decreased PN sensitivity (β-coefficient: -0.42, 95% confidence interval -0.72 to -0.12, P = .006). No association with treatment disruption was detected for the total number of missense variants or rs7349683.

CONCLUSION

Isolating toxicity sensitivity by accounting for exposure is a novel approach, and rs7349683 represents a promising marker for PN sensitivity that may be used to individualize paclitaxel treatment.

Abstract 4766: Transcriptomic profiling reveals a potential role for JAK/STAT inhibition in CDK4/6 inhibitor-resistant, ER+ breast cancers

Purpose: Specific cyclin-dependent kinase (CDK) inhibitors are standard of care for patients with metastatic, estrogen receptor-positive (ER+) breast cancer. CDK4/6 inhibitors have improved rates of progression free survival among metastatic, ER+ patients, but resistance limits their clinical efficacy. Various mechanisms of resistance to CDK4/6 inhibitors have been reported, but a comprehensive understanding of this resistance remains elusive.

Methods: We generated in vitro models of acquired (AR) and intrinsic (IR) resistance to CDK4/6 inhibitors using ER+ breast cancer cell lines (MCF-7, T47D) cultured with either continuous high dose (500nM) or dose-escalated (50nM to 500nM) CDK4/6 inhibition over three months. RNA expression and gene set enrichment analysis (GSEA) was used to nominate potential pathways associated with AR and IR palbociclib resistance. Reverse phase protein array (RPPA) and western blots were used to measure protein and phosphoprotein levels in CDK4/6 inhibitor resistant cell lines to validate nominated pathways. Cellular proliferation assays were performed to calculate the half-maximal inhibitory concentration (IC50) with inhibitors for CDK4/6 and JAK/STAT.

Results: Proliferation assays confirmed that MCF-7 AR and IR cells are resistant to palbociclib (IC50 both &gt;1uM) compared to parental cells (60nM); similar results were observed in the T47D cell lines. Cells resistant to either palbociclib, ribociclib, or abemaciclib demonstrated cross resistance to all three inhibitors. GSEA of transcriptomic data identified 579 genes (from AR cells) and 936 genes (from IR cells) that were differentially expressed between palbociclib-resistant MCF-7s and parental controls. RPPA analyses identified several key pathways that regulate CDK4/6 inhibitor resistance in these models. From GSEA analysis, the interferon (JAK/STAT) signaling pathway was the most differentially expressed pathway identified between palbociclib-resistant and sensitive cells. Western blot analyses showed that baseline expression of phospho-STAT1 is significantly elevated in palbociclib-resistant cells. In cellular proliferation assays, palbociclib-resistant MCF-7s and T47Ds retained sensitivity to JAK/STAT inhibitors like the JAK2-selective compound AZ960.

Conclusions: Our data suggests that overactivation of JAK/STAT signaling may be directly involved in the development of CDK4/6 inhibitor resistance in ER-dependent tumors. CDK4/6 inhibitor-resistant cells retain sensitivity to single-agent JAK/STAT inhibition, suggesting that this may be a viable therapeutic option for patients with CDK4/6 inhibitor-resistant ER+ breast cancer. This work was supported in part by 5T32GM007767-40 (Pesch), the Breast Cancer Research Foundation (N003173 to JMR), the UM Rogel Cancer Center and the Taubman Emerging Scholar funds.

Citation Format: Andrea M. Pesch, Thomas L. Gonzalez, Benjamin C. Chandler, Siqi Sun, Christina L. Gersch, José M. Larios, Wadie S. David, Corey W. Speers, James M. Rae. Transcriptomic profiling reveals a potential role for JAK/STAT inhibition in CDK4/6 inhibitor-resistant, ER+ breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4766.

Metabolites of n-Butylparaben and iso-Butylparaben Exhibit Estrogenic Properties in MCF-7 and T47D Human Breast Cancer Cell Lines

Two oxidized metabolites of n-butylparaben (BuP) and iso-butylparaben (IsoBuP) discovered in human urine samples exhibit structural similarity to endogenous estrogens. We hypothesized that these metabolites bind to the human estrogen receptor (ER) and promote estrogen signaling. We tested this using models of ER-mediated cellular proliferation. The estrogenic properties of 3-hydroxy n-butyl 4-hydroxybenzoate (3OH) and 2-hydroxy iso-butyl 4-hydroxybenzoate (2OH) were determined using the ER-positive, estrogen-dependent human breast cancer cell lines MCF-7, and T47D. The 3OH metabolite induced cellular proliferation with EC50 of 8.2 µM in MCF-7 cells. The EC50 for 3OH in T47D cells could not be reached. The 2OH metabolite induced proliferation with EC50 of 2.2 µM and 43.0 µM in MCF-7 and T47D cells, respectively. The EC50 for the parental IsoBuP and BuP was 0.30 and 1.2 µM in MCF-7 cells, respectively. The expression of a pro-proliferative, estrogen-inducible gene (GREB1) was induced by these compounds and blocked by co-administration of an ER antagonist (ICI 182, 780), confirming the ER-dependence of these effects. The metabolites promoted significant ER-dependent transcriptional activity of an ERE-luciferase reporter construct at 10 and 20 µM for 2OH and 10 µM for 3OH. Computational docking studies showed that the paraben compounds exhibited the potential for favorable ligand-binding domain interactions with human ERα in a manner similar to known x-ray crystal structures of 17ß-estradiol in complex with ERα. We conclude that the hydroxylated metabolites of BuP and IsoBuP are weak estrogens and should be considered as additional components of potential endocrine disrupting effects upon paraben exposure.

EPHA genetics for prediction of paclitaxel-induced peripheral neuropathy sensitivity

532

Background: Chemotherapy induced peripheral neuropathy (CIPN) is a common, debilitating paclitaxel side effect that has been primarily attributed to cumulative systemic paclitaxel exposure. Paclitaxel dose-adjustment to achieve target systemic exposure decreases but does not eliminate severe CIPN, suggesting some patients are inherently CIPN-sensitive. Ephrin ( EPHA) polymorphisms have been reported to increase CIPN occurrence (Baldwin 2012, Leandro-Garcia 2013, Boora 2016) but replication has been challenging, perhaps due to the inability to isolate CIPN-sensitivity by accounting for systemic exposure differences. The study purpose was to determine if EPHA genetic variants previously associated with CIPN occurrence are associated with CIPN-sensitivity. Methods: PN was assessed at baseline and weekly using the 8-item sensory subscale (CIPN8) of the patient-reported EORTC CIPN20 in patients receiving paclitaxel 80 mg/m2 weekly x 12. EPHA4, EPHA5, EPHA6, and EPHA8 were sequenced in germline DNA. Associations with higher PN-sensitivity were tested for three genetic models (total variants, coding variants, and rs7349683) by incorporating genetics into previously published CIPN8 predictive models that included measured paclitaxel exposure. Significant associations were tested for association with higher risk of PN-related treatment disruption (i.e. dose decrease, delay, or discontinuation). Results: In the 59 included patients, carrying EPHA5 rs7349683 was associated with greater CIPN sensitivity (beta coefficient: 0.40, standard error: 0.14, p = 0.005), indicating these patients had a greater increase in CIPN8 during treatment than would be predicted based on cumulative paclitaxel exposure. Rs7349683 was not associated with increased PN-induced treatment disruption, perhaps due to the low number of events (n = 19). Conclusions: Using a novel approach that isolates CIPN-sensitivity by accounting for measured paclitaxel exposure, our results provide further evidence that EPHA5 rs7349683 may be a promising marker for CIPN. If additional validation studies confirm this association, genetic testing could enable personalized treatment strategies to prevent CIPN in patients with breast cancer.

Impact of CYP3A5 phenotype on tacrolimus concentrations after sublingual and oral administration in lung transplant

Aim: This study evaluated the impact of CYP3A5 genotype and other patient characteristics on sublingual (SL) tacrolimus exposure and compared the relationship with oral administration. Patients & methods: Tacrolimus concentrations were retrospectively collected for adult lung transplant recipients, who were genotyped for CYP3A5*3, CYP3A4*22, CYP3A7*1C, and POR*28. Regression analyses were performed to determine covariates that impacted the SL and oral tacrolimus concentration/dose ratios. Results: An interaction of CYP3A5 genotype and CYP3A inhibitor increased the SL concentration/dose, while cystic fibrosis decreased the SL concentration/dose. The oral concentration/dose was independently associated with these covariates and was increased by serum creatinine and number of tacrolimus doses. Conclusion: This study suggests personalized dosing strategies for tacrolimus likely need to consider characteristics beyond CYP3A5 genotype.

Exemestane may be less detrimental than letrozole to bone health in women homozygous for the UGT2B17*2 gene deletion

PURPOSE

UGT2B17 gene deletion (UGT2B17*2) has been reported to affect bone health as well as the pharmacokinetics of aromatase inhibitor (AI) drugs such as exemestane. The goal of this study was to assess associations between UGT2B17 gene deletion and bone health prior to and after 24 months of AI treatment in postmenopausal women with hormone receptor positive (HR+) breast cancer.

METHODS

Bone health in women with HR+ breast cancer enrolled on the prospective randomized Exemestane and Letrozole Pharmacogenetics (ELPh) trial was determined by measuring bone turnover markers (BTM) and bone mineral density (BMD) pre-treatment and after 3 BTM and 24 BMD months of treatment with either the steroidal AI exemestane or the nonsteroidal AI letrozole. DNA samples were genotyped for UGT2B17*2.

RESULTS

Of the 455 subjects included in the analyses, 244 (53.6%) carried at least one copy of UGT2B17*2. UGT2B17*2 was associated with lower pre-treatment BMD at the hip (P = 0.01) and spine (P = 0.0076). Letrozole treatment was associated with a greater decrease in BMD of the hip (P = 0.03) and spine (P = 0.03) than exemestane. UGT2B17 genotype was not associated with changes in BMD from 24 months of AI treatment, though in UGT2B17*2 homozygous patients, there was a trend toward greater decreases in BMD of the spine from treatment with letrozole compared with exemestane (P = 0.05).

CONCLUSION

UGT2B17*2 may be associated with lower baseline BMD in women with HR+ breast cancer. Exemestane is less detrimental to bone health than letrozole in postmenopausal women treated with AI, and this effect may be confined to patients carrying UGT2B17*2, though this finding requires independent validation.

Variable aromatase inhibitor plasma concentrations do not correlate with circulating estrogen concentrations in post-menopausal breast cancer patients

PURPOSE

The aromatase inhibitors (AI) exemestane (EXE), letrozole (LET), and anastrozole suppress estrogen biosynthesis, and are effective treatments for estrogen receptor (ER)-positive breast cancer. Prior work suggests that anastrozole blood concentrations are associated with the magnitude of estrogen suppression. The objective of this study was to determine whether the magnitude of estrogen suppression, as determined by plasma estradiol (E2) concentrations, in EXE or LET treated patients is associated with plasma AI concentrations.

METHODS

Five hundred post-menopausal women with ER-positive breast cancer were enrolled in the prospective Exemestane and Letrozole Pharmacogenetic (ELPh) Study conducted by the COnsortium on BReast cancer phArmacogomics (COBRA) and randomly assigned to either drug. Estrogen concentrations were measured at baseline and after 3 months of AI treatment and drug concentrations were measured after 1 or 3 months. EXE or LET concentrations were compared with 3-month E2 concentration or the change from baseline to 3 months using several complementary statistical procedures.

RESULTS

Four-hundred patients with on-treatment E2 and AI concentrations were evaluable (EXE n = 200, LET n = 200). Thirty (7.6%) patients (EXE n = 13, LET n = 17) had 3-month E2 concentrations above the lower limit of quantification (LLOQ) (median: 4.75; range: 1.42-63.8 pg/mL). EXE and LET concentrations were not associated with on-treatment E2 concentrations or changes in E2 concentrations from baseline (all p > 0.05).

CONCLUSIONS

Steady-state plasma AI concentrations do not explain variability in E2 suppression in post-menopausal women receiving EXE or LET therapy, in contrast with prior evidence in anastrozole treated patients.

Is incomplete estradiol suppression during aromatase inhibitor treatment in post-menopausal patients with breast cancer due to insufficient systemic drug concentrations?

1063

Background: Aromatase inhibitors (AI) suppress estrogen biosynthesis and are effective treatments for estrogen receptor (ER)-positive breast cancer. In a prospectively enrolled cohort we observed a subset of post-menopausal women who exhibit high plasma estradiol (E2) concentrations during AI treatment, which could potentially contribute to treatment failure. We tested the hypothesis that incomplete E2 suppression is due to insufficient systemic AI concentrations. Methods: Five hundred post-menopausal women with ER-positive breast cancer were randomized to daily exemestane (Exe) 25 mg or letrozole (Let) 2.5 mg. Plasma E2 was measured using GC/MS/MS (lower limit of quantification (LLOQ) = 1.25 pg/mL) at baseline and after 3 months. Let and Exe plasma concentrations measured after 1 or 3 months were compared with the magnitude of E2 depletion using four complementary statistical procedures to assess associations of drug concentrations with: 1) a binary outcome of E2 suppression below LLOQ (logistic regression), 2) 3-month E2 concentrations (linear regression), 3) absolute change from baseline in E2 concentrations (Spearman correlation), and 4) an ordinal outcome defined by E2: decreased to below LLOQ, decreased but not to LLOQ, stayed the same, or increased from baseline (cumulative logistic regression). Results: 397 patients with E2 and AI concentration measurements were evaluable (Exe n = 199, Let n = 198). Thirty (7.6%) patients (Exe n = 13, Let n = 17) had E2 concentrations above the LLOQ at 3 months (range: 1.42-63.8 pg/mL). Exe and Let concentrations were not associated with achievement of unmeasurable E2 concentrations, on-treatment E2 concentrations, E2 change from baseline, or ordinal groupings of E2 change (all p > 0.05). In a parallel analysis there was no association of estrone-sulfate and drug concentrations (data not shown). Conclusions: Our results suggest that circulating drug concentrations do not explain incomplete E2 suppression in women receiving AI therapy. Additional studies are underway to determine whether age, body mass and genetic variation in the aromatase enzyme influence AI treatment response.

Abstract P6-09-11: Genetic variation in CYP3A affects steady-state exemestane concentrations but does not explain inter-race difference

Background: Exemestane is a third generation steroidal aromatase inhibitor (AI) used for the treatment of estrogen receptor (ER) positive breast cancer in postmenopausal women. Differences in AI treatment efficacy and side effects may be due, in part, to variability in drug exposure. We previously reported that patients who self-report as white and those who carry the low-activity CYP3A4*22 single nucleotide polymorphism (SNP) have increased exemestane steady-state concentrations. Additional SNPs in CYP3A may contribute to pharmacokinetic variability and explain this inter-race difference. CYP3A5*3 (rs776746) is a non-expresser genotype that is far more common in European (minor allele frequency (MAF)∼0.94) than African (MAF∼0.18) individuals. CYP3A7*1C (rs45446698) is believed to tag adult expression of the fetal CYP3A7 enzyme and is relatively uncommon in tested cohorts (European MAF=0.04, African MAF&lt;0.01). The objective of this secondary analysis was to determine whether these additional CYP3A SNPs contribute to variability in steady state exemestane concentrations and explain the inter-race difference.

Methods: 500 patients were randomly assigned to either drug on the Exemestane and Letrozole Pharmacogenetics (ELPh) Study. Clinical data and DNA were collected at baseline and blood samples were collected after 1 or 3 months of treatment to measure steady-state exemestane concentration via HPLC/MS. Genotyping for CYP3A5*3 and CYP3A7*1C was performed via Taqman Allelic Discrimination. Pharmacogenetic association with log-transformed concentrations were tested for each variant by inclusion in a multivariable model with CYP3A4*22 and self-reported race, assuming additive genetic effect, using Tobit regression to censor concentrations below the lower limit of quantification. SNPs with suggestive p-values &lt;0.10 were included in a multivariable model with relevant covariates (AST or ALT&gt;40, body mass index (BMI), and prior chemotherapy) to assess their independent contribution.

Results: In 231 evaluable patients there was a suggestive trend toward lower steady-state exemestane concentrations for CYP3A7*1C carriers (6.3 vs. 8.0 ng/mL) in the model including CYP3A4*22 and race (p=0.083). In the final multivariable model each CYP3A7*1C allele decreased exemestane concentration 31.5% (p=0.035, Table 1). CYP3A5*3 was not associated with exemestane concentration (p&gt;0.2).

Multivariable Model of Exemestane Concentration % change in concentration (95% CI)p-valueCYP3A4*22 (rs35599367)64.5% (23%, 120%)0.0008CYP3A7*1C (rs45446698)-31.5% (-52%, -2.6%)0.035Self-Reported White47.2% (9.0%, 99%)0.012AST or ALT&gt;4041.3% (1.0%, 98%)0.044BMI-0.9% (-2.4%, 0.55%)0.22Prior Chemotherapy-23.5% (-37%, -7.6%)0.006CI: Confidence Interval

Conclusions: Patients with breast cancer who carry CYP3A7*1C have lower steady-state exemestane concentrations but this association does not explain the greater concentrations in self-reported white patients. Ongoing analyses will determine whether exemestane concentration predicts treatment efficacy or toxicity, and if so, whether genetic and clinical factors can be useful for individualizing dosing to optimize outcomes. CYP3A7*1C should be prioritized for analyses of pharmacokinetic variability of other CYP3A substrates.

Citation Format: Hertz DL, Kidwell KM, Gersch CL, Desta Z, Storniolo AM, Stearns V, Skaar TC, Hayes DF, Henry NL, Rae JM. Genetic variation in CYP3A affects steady-state exemestane concentrations but does not explain inter-race difference [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-09-11.

Abstract P5-12-05: CYP3A4*22 polymorphism is associated with increased exemestane concentrations in postmenopausal breast cancer patients

Background: Exemestane is a second generation steroidal aromatase inhibitor (AI) used for the treatment of estrogen receptor (ER) positive breast cancer in postmenopausal women. Variability in AI treatment efficacy and side effects seen across patients may be due, in part, to inter-patient differences in drug exposure. This exposure variability is likely caused by patient genetics factors, such as single nucleotide polymorphisms (SNPs) in drug metabolizing enzymes, or clinical factors such as patient body size, organ function, and comorbidities. The objective of this secondary correlative analysis was to identify genetic and clinical characteristics that affect steady state exemestane concentration, with a specific focus on the influence of inherited genetic variants and baseline hepatic function.

Methods: 500 patients were enrolled on the Exemestane and Letrozole Pharmacogenetics (ELPh) Study and randomized to either drug. Clinical data and DNA were collected at baseline and blood samples were collected after 1 or 3 months of treatment to measure steady-state exemestane concentration via HPLC/MS. Genotyping was performed on a custom Sequenom MassARRAY iPLEX that included the recently discovered low activity CYP3A4*22 (rs35599367) SNP and several other SNPs with putative functional consequence in enzymes thought to be involved in exemestane metabolism (CYP1A1/2, CYP1B1, CYP3A4, CYP4A11, AKR1C3/4, AKR7A2). Our primary hypothesis was that patients carrying CYP3A4*22 variants would have higher serum exemestane concentrations. Other SNPs and clinical characteristics (hepatic and renal function, age, body mass index (BMI), time of sample collection, prior chemotherapy) were assessed for independent association, and then adjusted for in a multivariable tobit regression model for CYP3A4*22 on log-transformed censored exemestane concentration.

Results: 246 (225 randomized to exemestane arm, 21 crossed-over from letrozole arm) patients had exemestane steady state levels and were evaluable in this analysis. As hypothesized, the CYP3A4*22 polymorphism (minor allele frequency=0.06) was associated with a 54% increase in exemestane concentration (95% CI: 14% - 109%, p&lt;0.01). Exemestane concentration was 44% greater in patients who had evidence of hepatic impairment (AST or ALT&gt;40) at baseline (95% CI: 2% - 104%, p=0.02), 1% lower per unit increase in BMI (95% CI: 0% - 3%, p=0.05), and 20% lower in patients who received prior chemotherapy (95% CI: 4% - 34%, p=0.03). Age, renal impairment, and other SNPs were not associated with exemestane concentration. After adjustment for significant clinical covariates the CYP3A4*22 SNP remained significant (p&lt;0.01).

Conclusions: Genetic and clinical predictors of exemestane concentration were discovered in a large cohort of prospectively enrolled estrogen responsive breast cancer patients. Ongoing analyses will determine whether the variability in exemestane concentration was associated with downstream effects on estrogen depletion or treatment-related toxicity. If so, these genetic and clinical characteristics could be useful for individualizing dosing of exemestane to ensure that all patients are receiving maximal benefit with minimal toxicity.

Citation Format: Hertz DL, Kidwell KM, Seewald NJ, Gersch CL, Desta Z, Flockhart DA, Storniolo AM, Stearns V, Skaar TC, Hayes DF, Henry NL, Rae JM. CYP3A4*22 polymorphism is associated with increased exemestane concentrations in postmenopausal breast cancer patients. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P5-12-05.