An Analysis of Recent FDA Oncology Scientific Publications.

In addition to its primary regulatory role, the Office of Hematology and Oncology Products at the U.S. Food and Drug Administration (FDA) is engaged in many forms of scientific authorship. During the period of 2010 to 2018, FDA oncology staff contributed to 356 publications in the scientific literature. Here, we collaborated with analysts in the Office of Program Planning, Analysis, and Evaluation at the National Institute of General Medical Sciences, National Institutes of Health (NIH), to present a series of analyses aimed at quantifying the characteristics and potential impact of these contributions, as well as characterizing the areas of work addressed. We found that FDA oncology papers are enriched for high-impact publications and have about two times the number of citations as an average NIH-funded paper. Further impact of the publications was measured based on the presence of 65 publications that were cited by guidelines and 12 publications cited by publicly listed clinical trials. The results seen here are promising in determining the impact of FDA oncology publication work but prompt further investigation into longer-term impacts, such as the influence of this work on other regulatory activities at FDA. IMPLICATIONS FOR PRACTICE: This article describes the first comprehensive study of scientific publications produced by U.S. Food and Drug Administration (FDA) oncology staff. The analysis illustrates that staff are highly engaged in publishing in the scientific literature in addition to completing regulatory review work. Publications are generally in clinical medicine, consistent with the large number of medical oncologists working at the Office of Hematology and Oncology Products (OHOP). OHOP publications generally focus either on communicating important regulatory work (approval summaries) or highlighting regulatory science issues to encourage dialogue with the scientific community (commentaries, reviews, and expert working papers). The analysis also suggests that several FDA oncology publications may influence clinical guidelines, but further work is needed to evaluate impact.

Using database linkages to measure innovation, commercialization, and survival of small businesses.

Here, we report the results of an outcomes evaluation of the Small Business Innovation Research (SBIR) and Small Business Technology Transfer (STTR) Programs at the National Institute of General Medical Sciences (NIGMS). Since the programs' inception, assessments of the SBIR/STTR programs at several federal agencies have utilized surveys of former grantees as the primary source of data. Response rates have typically been low, making non-response bias a potential threat to the validity of some of these studies' results. Meanwhile, the availability of large publicly-available datasets continues to grow and methods of text mining and linking databases continue to improve. By linking NIGMS grant funding records, U.S. Patent and Trademark Office data, and business intelligence databases, we explored innovation, commercialization and survival for recipients of NIGMS SBIR/STTR funding. In doing so, we were able to more completely assess several key outcomes of the NIGMS SBIR/STTR program. Our evaluation demonstrated that the NIGMS program performed above baseline expectations along all dimensions, and comparably to other federal agency SBIR/STTR grant programs. In addition, we show that the use of extant data increasingly is a viable, less expensive, and more reliable approach to gathering data for evaluation studies.

Geographically-related outcomes of U.S. funding for small business research and development: Results of the research grant programs of a component of the National Institutes of Health.

This article examines the geographic distribution of funding for the U.S. Small Business Innovation Research (SBIR) and Small Business Technology Transfer (STTR) programs sponsored by the National Institute of General Medical Sciences (NIGMS). Despite a significant investment in SBIR/STTR and an interest in increasing geographic diversity in the institute's research portfolio, there has not been an assessment of the distribution of NIGMS's SBIR/STTR funding, outcomes associated with that investment, and relationships between the two. The geographic distribution of NIGMS' SBIR/STTR funding was highly concentrated in a small number of states, with a high correlation between each state's funding and its number of small scientific research and development businesses. Affiliation with a major research university was correlated with several measures of innovation and firm success. Our findings are consistent with earlier research showing that economic activity in research and development and research output tend to cluster in geographic regions where knowledge can be generated and shared more efficiently. These findings lend support to an investment strategy for small business research and development that creates networks between major research universities and small businesses.

NIH funding longevity by gender.

Women have achieved parity with men among biomedical science degree holders but remain underrepresented in academic positions. The National Institutes of Health (NIH)-the world's largest public funder of biomedical research-receives less than one-third of its new grant applications from women. Correspondingly, women compose less than one-third of NIH research grantees, even though they are as successful as men in obtaining first-time grants. Our study examined women's and men's NIH funding trajectories over time (n = 34,770), exploring whether women remain funded at the same rate as men after receiving their first major research grants. A survival analysis demonstrated a slightly lower funding longevity for women. We next examined gender differences in application, review, and funding outcomes. Women individually held fewer grants, submitted fewer applications, and were less successful in renewing grants-factors that could lead to gender differences in funding longevity. Finally, two adjusted survival models that account for initial investigator characteristics or subsequent application behavior showed no gender differences, suggesting that the small observed longevity differences are affected by both sets of factors. Overall, given men's and women's generally comparable funding longevities, the data contradict the common assumption that women experience accelerated attrition compared with men across all career stages. Women's likelihood of sustaining NIH funding may be better than commonly perceived. This suggests a need to explore women's underrepresentation among initial NIH grantees, as well as their lower rates of new and renewal application submissions.

A Gene Expression Signature Associated with Overall Survival in Patients with Hepatocellular Carcinoma Suggests a New Treatment Strategy.

Despite improvements in the management of liver cancer, the survival rate for patients with hepatocellular carcinoma (HCC) remains dismal. The survival benefit of systemic chemotherapy for the treatment of liver cancer is only marginal. Although the reasons for treatment failure are multifactorial, intrinsic resistance to chemotherapy plays a primary role. Here, we analyzed the expression of 377 multidrug resistance (MDR)-associated genes in two independent cohorts of patients with advanced HCC, with the aim of finding ways to improve survival in this poor-prognosis cancer. Taqman-based quantitative polymerase chain reaction revealed a 45-gene signature that predicts overall survival (OS) in patients with HCC. Using the Connectivity Map Tool, we were able to identify drugs that converted the gene expression profiles of HCC cell lines from ones matching patients with poor OS to profiles associated with good OS. We found three compounds that convert the gene expression profiles of three HCC cell lines to gene expression profiles associated with good OS. These compounds increase histone acetylation, which correlates with the synergistic sensitization of those MDR tumor cells to conventional chemotherapeutic agents, including cisplatin, sorafenib, and 5-fluorouracil. Our results indicate that it is possible to modulate gene expression profiles in HCC cell lines to those associated with better outcome. This approach also increases sensitization of HCC cells toward conventional chemotherapeutic agents. This work suggests new treatment strategies for a disease for which few therapeutic options exist.

Role of KRAS-LCS6 polymorphism in advanced NSCLC patients treated with erlotinib or docetaxel in second line treatment (TAILOR).

MicroRNAs were described to target mRNA and regulate the transcription of genes involved in processes de-regulated in tumorigenesis, such as proliferation, differentiation and survival. In particular, the miRNA let-7 has been suggested to regulate the expression of the KRAS gene, a common mutated gene in non-small cell lung cancer (NSCLC), through a let-7 complementary site (LCS) in 3'UTR of KRAS mRNA. We have reported the analysis performed on the role of the polymorphism located in the KRAS-LCS (rs61764370) which is involved in the disruption of the let-7 complementary site in NSCLC patients enrolled within the TAILOR trial, a randomised trial comparing erlotinib versus docetaxel in second line treatment. In our cohort of patients, KRAS-LCS6 polymorphism did not have any impact on both overall survival (OS) and progression free survival (PFS) and was not associated with any patient's baseline characteristics included in the study. Overall, patients had a better prognosis when treated with docetaxel instead of erlotinib for both OS and PFS. Considering KRAS-LCS6 status, the TG/GG patients had a benefit from docetaxel treatment (HR(docetaxel vs erlotinib) = 0.35, 95% CI 0.15-0.79, p = 0.011) compared with the TT patients (HR(docetaxel vs erlotinib) = 0.72, 95% CI 0.52-1.01, p = 0.056) in terms of PFS.

Colorectal Cancer with Uncommon Metastatic Spread.

The prevalence of bone metastases from colorectal cancer (CRC) is quite low and the presence of isolated osseous metastases at the time of diagnosis or the onset of bone metastases without other organ involvement during follow-up is even lower. Here, we present an interesting case of diffuse skeletal metastases from CRC in which both the atypical presentation of the metastatic spread and the presence of infective comorbidities created some troubles in getting the final diagnosis.

Multidrug resistance-linked gene signature predicts overall survival of patients with primary ovarian serous carcinoma.

PURPOSE: This study assesses the ability of multidrug resistance (MDR)-associated gene expression patterns to predict survival in patients with newly diagnosed carcinoma of the ovary. The scope of this research differs substantially from that of previous reports, as a very large set of genes was evaluated whose expression has been shown to affect response to chemotherapy. EXPERIMENTAL DESIGN: We applied a customized TaqMan low density array, a highly sensitive and specific assay, to study the expression profiles of 380 MDR-linked genes in 80 tumor specimens collected at initial surgery to debulk primary serous carcinoma. The RNA expression profiles of these drug resistance genes were correlated with clinical outcomes. RESULTS: Leave-one-out cross-validation was used to estimate the ability of MDR gene expression to predict survival. Although gene expression alone does not predict overall survival (OS; P = 0.06), four covariates (age, stage, CA125 level, and surgical debulking) do (P = 0.03). When gene expression was added to the covariates, we found an 11-gene signature that provides a major improvement in OS prediction (log-rank statistic P < 0.003). The predictive power of this 11-gene signature was confirmed by dividing high- and low-risk patient groups, as defined by their clinical covariates, into four specific risk groups on the basis of expression levels. CONCLUSION: This study reveals an 11-gene signature that allows a more precise prognosis for patients with serous cancer of the ovary treated with carboplatin- and paclitaxel-based therapy. These 11 new targets offer opportunities for new therapies to improve clinical outcome in ovarian cancer.

Redefining the relevance of established cancer cell lines to the study of mechanisms of clinical anti-cancer drug resistance.

Although in vitro models have been a cornerstone of anti-cancer drug development, their direct applicability to clinical cancer research has been uncertain. Using a state-of-the-art Taqman-based quantitative RT-PCR assay, we investigated the multidrug resistance (MDR) transcriptome of six cancer types, in established cancer cell lines (grown in monolayer, 3D scaffold, or in xenograft) and clinical samples, either containing >75% tumor cells or microdissected. The MDR transcriptome was determined a priori based on an extensive curation of the literature published during the last three decades, which led to the enumeration of 380 genes. No correlation was found between clinical samples and established cancer cell lines. As expected, we found up-regulation of genes that would facilitate survival across all cultured cancer cell lines evaluated. More troubling, however, were data showing that all of the cell lines, grown either in vitro or in vivo, bear more resemblance to each other, regardless of the tissue of origin, than to the clinical samples they are supposed to model. Although cultured cells can be used to study many aspects of cancer biology and response of cells to drugs, this study emphasizes the necessity for new in vitro cancer models and the use of primary tumor models in which gene expression can be manipulated and small molecules tested in a setting that more closely mimics the in vivo cancer microenvironment so as to avoid radical changes in gene expression profiles brought on by extended periods of cell culture.

Clinical relevance of multidrug resistance gene expression in ovarian serous carcinoma effusions.

The presence of tumor cells in effusions within serosal cavities is a clinical manifestation of advanced-stage cancer and is generally associated with poor survival. Identifying molecular targets may help to design efficient treatments to eradicate these aggressive cancer cells and improve patient survival. Using a state-of-the-art TaqMan-based qRT-PCR assay, we investigated the multidrug resistance (MDR) transcriptome of 32 unpaired ovarian serous carcinoma effusion samples obtained at diagnosis or at disease recurrence following chemotherapy. MDR genes were selected a priori based on an extensive curation of the literature published during the last three decades. We found three gene signatures with a statistically significant correlation with overall survival (OS), response to treatment [complete response (CR) vs other], and progression free survival (PFS). The median log-rank p-values for the signatures were 0.023, 0.034, and 0.008, respectively. No correlation was found with residual tumor status after cytoreductive surgery, treatment (with or without chemotherapy) and stage defined according to the International Federation of Gynecology and Obstetrics. Further analyses demonstrated that gene expression alone can effectively predict the survival outcome of women with ovarian serous carcinoma (OS, log-rank p = 0.0000; and PFS, log-rank p = 0.002). Interestingly, the signature for overall survival is the same in patients at first presentation and those who had chemotherapy and relapsed. This pilot study highlights two new gene signatures that may help in optimizing the treatment for ovarian carcinoma patients with effusions.

Sequential use of sorafenib and sunitinib in advanced renal-cell carcinoma (RCC): an Italian multicentre retrospective analysis of 189 patient cases.

OBJECTIVE: •To conduct a retrospective, multicentre, cohort analysis to assess the sequential use of the tyrosine kinase inhibitors (TKIs) sorafenib and sunitinib. PATIENTS AND METHODS: •Records of 189 patients with renal-cell carcinoma (RCC) who were treated with sorafenib and sunitinib sequentially between March 2004 and April 2009 at 12 Italian study centres were analysed. •Patients were treated under European Expanded Access Programmes or, following market approval, in general clinical practice. •Interventions were sorafenib (800 mg/day) and sunitinib (50 mg every day; 4 weeks on and 2 weeks off). •Progression-free survival (PFS) during treatment with the first and second TKI was evaluated. RESULTS: •In all, 99 patients were treated with sunitinib followed by sorafenib (SuSo) and 90 were treated with sorafenib followed by sunitinib (SoSu); 104 (55%) patients had received prior systemic therapy, mostly with cytokines. •The median (range) PFS on the first TKI was similar between treatment groups [sorafenib 8.4 (1.1-28.9) months; sunitinib 7.8 (0.5-30.4) months; hazard ratio (HR) 1.05, 95% confidence interval (CI) 0.78-1.40, P=0.758]. Multivariate analysis showed that good Memorial Sloan-Kettering Cancer Center status was associated with increased PFS. •After the second TKI, patients in the SoSu group had a longer median PFS than those in the SuSo group (7.9 months vs 4.2 months, respectively; HR 0.54, 95% CI 0.39-0.74, P<0.001). •Multivariate analysis showed only treatment and Eastern Cooperative Oncology Group performance status (and not age, gender, study centre or previous treatment) were significantly associated with duration of PFS. CONCLUSION: •Our findings suggest a limited cross-resistance between sorafenib and sunitinib and that the sequence SoSu may result in a longer combined PFS than SuSo. This is the largest retrospective study to date, though its findings are limited in part by the retrospective nature.

Enhancement of drug absorption through the blood-brain barrier and inhibition of intercellular tight junction resealing by E-cadherin peptides.

E-cadherin-mediated cell-cell interactions in the zonula adherens play an important role in the formation of the intercellular tight junctions found in the blood-brain barrier. However, it is also responsible for the low permeation of drugs into the brain. In this study, HAV6 peptide derived from the EC1 domain of E-cadherin was found to enhance the permeation of ¹4C-mannitol and [³H(G)]-daunomycin through the blood-brain barrier of the in situ rat brain perfusion model. In addition, HAV6 peptide and verapamil have a synergistic effect in enhancing the BBB permeation of daunomycin. A new intercellular-junction resealing assay was also developed using Caco-2 monolayers to evaluate new peptides (BLG2, BLG3, and BLG4) derived from the bulge regions of the EC2, EC3, and EC4 domains of E-cadherin. BLG2 and BLG4 peptides but not BLG3 peptides were found to be effective in blocking the resealing of the intercellular junctions. The positive control peptides (ADT10, ADT6, and HAV10) block the resealing of the intercellular junctions in a concentration-dependent manner. All these findings suggest that E-cadherin-derived peptides can block E-cadherin-mediated cell-cell interactions. These findings demonstrate that cadherin peptides may offer a useful targeted permeation enhancement of therapeutic agents such as anticancer drugs into the brain.

Prolonged drug selection of breast cancer cells and enrichment of cancer stem cell characteristics.

BACKGROUND: Cancer stem cells are presumed to have virtually unlimited proliferative and self-renewal abilities and to be highly resistant to chemotherapy, a feature that is associated with overexpression of ATP-binding cassette transporters. We investigated whether prolonged continuous selection of cells for drug resistance enriches cultures for cancer stem-like cells. METHODS: Cancer stem cells were defined as CD44+/CD24⁻ cells that could self-renew (ie, generate cells with the tumorigenic CD44+/CD24⁻ phenotype), differentiate, invade, and form tumors in vivo. We used doxorubicin-selected MCF-7/ADR cells, weakly tumorigenic parental MCF-7 cells, and MCF-7/MDR, an MCF-7 subline with forced expression of ABCB1 protein. Cells were examined for cell surface markers and side-population fractions by microarray and flow cytometry, with in vitro invasion assays, and for ability to form mammospheres. Xenograft tumors were generated in mice to examine tumorigenicity (n = 52). The mRNA expression of multidrug resistance genes was examined in putative cancer stem cells and pathway analysis of statistically significantly differentially expressed genes was performed. All statistical tests were two-sided. RESULTS: Pathway analysis showed that MCF-7/ADR cells express mRNAs from ABCB1 and other genes also found in breast cancer stem cells (eg, CD44, TGFB1, and SNAI1). MCF-7/ADR cells were highly invasive, formed mammospheres, and were tumorigenic in mice. In contrast to parental MCF-7 cells, more than 30% of MCF-7/ADR cells had a CD44+/CD24⁻ phenotype, could self-renew, and differentiate (ie, produce CD44+/CD24⁻ and CD44+/CD24+ cells) and overexpressed various multidrug resistance-linked genes (including ABCB1, CCNE1, and MMP9). MCF-7/ADR cells were statistically significantly more invasive in Matrigel than parental MCF-7 cells (MCF-7 cells = 0.82 cell per field and MCF-7/ADR = 7.51 cells per field, difference = 6.69 cells per field, 95% confidence interval = 4.82 to 8.55 cells per field, P < .001). No enrichment in the CD44+/CD24⁻ or CD133+ population was detected in MCF-7/MDR. CONCLUSION: The cell population with cancer stem cell characteristics increased after prolonged continuous selection for doxorubicin resistance.

Analysis of expression of drug resistance-linked ABC transporters in cancer cells by quantitative RT-PCR.

Quantitative real-time PCR (qRT-PCR) boasts many advantages over microarrays. For instance, very low amounts of total RNA are required to yield highly accurate and reproducible detection of transcript levels. As a consequence, qRT-PCR has become a popular technique for assessing gene expression levels and is now the gold standard. In this chapter, qRT-PCR using two distinct chemistries, SYBR Green and TaqMan, are described. We compare ABC transporter levels in various drug-resistant cancer cell lines by employing each method. SYBR Green yields reproducible results; nevertheless, TaqMan chemistry is superior to SYBR Green, as it displays higher specificity and sensitivity. Gene expression analysis by qRT-PCR is a powerful technique and shows potential as a diagnostic tool for predicting drug response in cancer patients.

Molecular mechanisms of drug resistance in single-step and multi-step drug-selected cancer cells.

Multidrug resistance (MDR) remains one of the key determinants in chemotherapeutic success of cancer patients. Often, acquired resistance is mediated by the overexpression of ATP-binding cassette (ABC) drug transporters. To study the mechanisms involved in the MDR phenotype, investigators have generated a variety of in vitro cell culture models using both multi-step and single-step drug selections. Sublines produced from multi-step selections have led to the discovery of several crucial drug transporters including ABCB1, ABCC1, and ABCG2. Additionally, a number of mechanisms causing gene overexpression have been elucidated. To more closely mimic in vivo conditions, investigators have also established MDR sublines with single-step drug selections. Here, we examine some of the multi-step and single-step selected cell lines generated to elucidate the mechanisms involved in the development of MDR in cancer cells.

Evaluation of current methods used to analyze the expression profiles of ATP-binding cassette transporters yields an improved drug-discovery database.

The development of multidrug resistance (MDR) to chemotherapy remains a major challenge in the treatment of cancer. Resistance exists against every effective anticancer drug and can develop by multiple mechanisms. These mechanisms can act individually or synergistically, leading to MDR, in which the cell becomes resistant to a variety of structurally and mechanistically unrelated drugs in addition to the drug initially administered. Although extensive work has been done to characterize MDR mechanisms in vitro, the translation of this knowledge to the clinic has not been successful. Therefore, identifying genes and mechanisms critical to the development of MDR in vivo and establishing a reliable method for analyzing highly homologous genes from small amounts of tissue is fundamental to achieving any significant enhancement in our understanding of MDR mechanisms and could lead to treatments designed to circumvent it. In this study, we use a previously established database that allows the identification of lead compounds in the early stages of drug discovery that are not ATP-binding cassette (ABC) transporter substrates. We believe this can serve as a model for appraising the accuracy and sensitivity of current methods used to analyze the expression profiles of ABC transporters. We found two platforms to be superior methods for the analysis of expression profiles of highly homologous gene superfamilies. This study also led to an improved database by revealing previously unidentified substrates for ABCB1, ABCC1, and ABCG2, transporters that contribute to MDR.

Reversal of ABC drug transporter-mediated multidrug resistance in cancer cells: evaluation of current strategies.

Overexpression of ATP-binding cassette (ABC) drug transporters that actively efflux a variety of amphipathic compounds can cause multidrug resistance (MDR) in cancer cells, which is a major obstacle in the success of cancer chemotherapy. The development of synthetic small molecule compounds or the identification of natural products that block ABC transporter-mediated efflux has been the conventional approach used to combat MDR. The strategy of using chemosensitizers, however, has not been successful in clinical cancer chemotherapy. Therefore, alternative approaches to identify or to synthesize compounds that can induce selective toxicity in cancer cells overexpressing one or more ABC transporters have been undertaken. This review summarizes the recent advances in identifying strategies to restore sensitivity to chemotherapeutics in multidrug resistant cancer cells.

Brca1 breast tumors contain distinct CD44+/CD24- and CD133+ cells with cancer stem cell characteristics.

INTRODUCTION: Whether cancer stem cells occur in BRCA1-associated breast cancer and contribute to therapeutic response is not known. METHODS: We generated and characterized 16 cell lines from five distinct Brca1deficient mouse mammary tumors with respect to their cancer stem cell characteristics. RESULTS: All cell lines derived from one tumor included increased numbers of CD44+/CD24- cells, which were previously identified as human breast cancer stem cells. All cell lines derived from another mammary tumor exhibited low levels of CD44+/CD24- cells, but they harbored 2% to 5.9% CD133+ cells, which were previously associated with cancer stem cells in other human and murine tumors. When plated in the absence of attachment without presorting, only those cell lines that were enriched in either stem cell marker formed spheroids, which were further enriched in cells expressing the respective cancer stem cell marker. In contrast, cells sorted for CD44+/CD24- or CD133+ markers lost their stem cell phenotype when cultured in monolayers. As few as 50 to 100 CD44+/CD24- or CD133+ sorted cells rapidly formed tumors in nonobese diabetic/severe combined immunodeficient mice, whereas 50-fold to 100-fold higher numbers of parental or stem cell depleted cells were required to form few, slow-growing tumors. Expression of stem cell associated genes, including Oct4, Notch1, Aldh1, Fgfr1, and Sox1, was increased in CD44+/CD24- and CD133+ cells. In addition, cells sorted for cancer stem cell markers and spheroid-forming cells were significantly more resistant to DNA-damaging drugs than were parental or stem cell depleted populations, and they were sensitized to the drugs by the heat shock protein-90 inhibitor 17-DMAG (17-dimethylaminoethylamino-17-demethoxygeldanamycin hydrochloride). CONCLUSION: Brca1-deficient mouse mammary tumors harbor heterogeneous cancer stem cell populations, and CD44+/CD24- cells represent a population that correlates with human breast cancer stem cells.

Evidence for dual mode of action of a thiosemicarbazone, NSC73306: a potent substrate of the multidrug resistance linked ABCG2 transporter.

Multidrug resistance due to reduced drug accumulation is a phenomenon predominantly caused by the overexpression of members of the ATP-binding cassette (ABC) transporters, including ABCB1 (P-glycoprotein), ABCG2, and several ABCC family members [multidrug resistance-associated protein (MRP)]. We previously reported that a thiosemicarbazone derivative, NSC73306, is cytotoxic to carcinoma cells that overexpress functional P-glycoprotein, and it resensitizes these cells to chemotherapeutics. In this study, we investigated the effect of NSC73306 on cells overexpressing other ABC drug transporters, including ABCG2, MRP1, MRP4, and MRP5. Our findings showed that NSC73306 is not more toxic to cells that overexpress these transporters compared with their respective parental cells, and these transporters do not confer resistance to NSC73306 either. In spite of this, we observed that NSC73306 is a transport substrate for ABCG2 that can effectively inhibit ABCG2-mediated drug transport and reverse resistance to both mitoxantrone and topotecan in ABCG2-expressing cells. Interactions between NSC73306 and the ABCG2 drug-binding site(s) were confirmed by its stimulatory effect on ATPase activity (140-150 nmol/L concentration required for 50% stimulation) and by inhibition of [(125)I]iodoarylazidoprazosin photolabeling (50% inhibition at 250-400 nmol/L) of the substrate-binding site(s). Overall, NSC73306 seems to be a potent modulator of ABCG2 that does not interact with MRP1, MRP4, or MRP5. Collectively, these data suggest that NSC73306 can potentially be used, due to its dual mode of action, as an effective agent to overcome drug resistance by eliminating P-glycoprotein-overexpressing cells and by acting as a potent modulator that resensitizes ABCG2-expressing cancer cells to chemotherapeutics.