We developed a systematic algorithmic solution for quantitative drug sensitivity scoring (DSS), based on continuous modeling and integration of multiple dose-response relationships in high-throughput compound testing studies. Mathematical model estimation and continuous interpolation makes the scoring approach robust against sources of technical variability and widely applicable to various experimental settings, both in cancer cell line models and primary patient-derived cells. Here, we demonstrate its improved performance over other response parameters especially in a leukemia patient case study, where differential DSS between patient and control cells enabled identification of both cancer-selective drugs and drug-sensitive patient sub-groups, as well as dynamic monitoring of the response patterns and oncogenic driver signals during cancer progression and relapse in individual patient cells ex vivo. An open-source and easily extendable implementation of the DSS calculation is made freely available to support its tailored application to translating drug sensitivity testing results into clinically actionable treatment options.
Cell-based compound screening provides a rich functional readout for many biomedical applications. In cancer research, the possibility to profile cellular responses to an extensive collection of anti-cancer compounds enables a systematic means to repurpose existing drugs to new indications, identify druggable vulnerabilities in various types of cancer cells and to functionally investigate cellular pathways behind drug sensitivity or resistance. Recent studies have successfully explained or even predicted drug responses by means of genetic aberrations or other genomic biomarkers in wide panels of cancer cell lines1,2,3,4. Similar large-scale drug testing efforts in primary cancer samples are increasingly being carried out to enable functional investigation of cellular addictions in individual cancer patients; for instance, to predict pathway dependencies and to identify potential therapeutic options for leukemia patients5,6. Systematic profiling of the relative activity of hundreds or thousands of drugs at several concentrations in a large number of cancer samples or cell types facilitates the stratification of cancer patients and disease subtypes, as well as development of personalized treatment strategies for clinical applications.
However, high-throughput drug testing experiments often result in high-dimensional sample-dose-response matrices, with inherent measurement noise and technical variability, which hinders many downstream analyses, such as those aimed at detecting differential drug sensitivities or clustering of patients and/or drugs based on their selective response patterns. To provide quantitative information about the degree of drug efficacy in a given sample, the dose dimension of these matrices is often summarized into single response parameter estimated form dose-response models, such as IC50 or EC50 (half-maximal inhibitory/effective concentration)1,2,3,5,7,8. Although proven sufficient in many applications, any single model parameter can capture only limited information about the differences in the response patterns9, especially when comparing cancer and normal cells. Recently, an ‘Activity Area’ metric was used to estimate both the efficacy and potency of 24 compounds in hundreds of cancer cell lines3. This type of discrete approximation, based on summing up the observed responses at each dose level, was shown to perform well under controlled in vitro settings with relatively densely-sampled concentration ranges and narrow bioactivity spectra3.
Here, we developed and implemented a quantitative scoring approach, named drug sensitivity score (DSS), which captures and integrates the multiparametric dose-response relationships into a single metric to identify selective drug response patterns between cancer and control cells, rather than scoring drug activity in cancer cells alone. Analytic integration of the area under the non-linear dose-response model combines the advantages of both the model-based and area-based response calculations. Applications of DSS to drug sensitivity testing of acute myeloid leukemia (AML) patient cells ex vivo demonstrated its improved performance, also when profiling larger compound panels and broader bioactivity spectra at sparsely-sampled dose levels (10,000-fold range) in fresh, primary cells. Several case studies in in vitro models from the Cancer Cell Line Encyclopedia (CCLE) resource3 also supported the applicability of the DSS metric to various experimental settings and application cases, where the aim is to identify both sensitive and selective drug response patterns. To promote its application to the future drug testing studies, we have made publicly available an open-source and easily extendable implementation of the model-based DSS calculations in the form of a stand-alone R-package.
Our quantitative scoring approach is based on closed-form integration of the area under the estimated dose-response curve (AUC; Figure 1a); the generic modeling approach can be used in the context of standard logistic, sigmoidal or Hill slope response functions (Figure 1b). The continuous model estimation and interpolation effectively summarize the complex dose-response relationship into a single response metric, named DSS (Supplementary Fig. 1b). More formally, if R(x) models the normalized drug response at a concentration x, then the integral response I over the dose range that exceeds a given minimum activity level, Amin, is calculated analytically as a continuous function of multiple parameters of the non-linear response model, including its slope at IC50 as well as the top and bottom asymptotes of the response (Rmax and Rmin).: Importantly, differential DSS (dDSS) quantifies the selective response of cancer cells, relative to that of control cells, when control samples are available; dDSS is calculated by the difference between drug response quantified in patient cells (patient DSS) and the average drug response of control samples (controls DSS) (Figure 1c). To discriminate those compounds which are effective at higher concentrations only (potential toxic off-target responses), and to favor those that show potency over a relative wide therapeutic window, the analytic AUC calculation (referred to as DSS1) was further normalized by the logarithm of the top asymptote Rmax(DSS2) and by the dose range over which the response exceeds the activity threshold Amin (DSS3), respectively (mathematical derivation of the closed-form solutions when using four-parameter logistic response model is given in Supplementary Methods). The DSS R-package and its source code are freely available at https://dss-calculation.googlecode.com/svn/trunk/.
DSS calculation improves drug response profiling in primary leukemic cells
We initially developed and implemented the DSS calculation in the context of our ongoing drug sensitivity and resistance testing (DSRT) program, with the aim to provide informed choices for clinicians on the treatment of relapsed or chemorefractory acute myeloid leukemia (AML) patients based on the ex vivo DSRT results of the patient cells6. The screening panel of 204 compounds used in this study covers virtually all FDA-approved small molecule anti-cancer drugs, along with a collection of emerging, investigational and preclinical oncology compounds, including signal transduction inhibitors targeting major oncogenic signaling pathways (Supplementary Table 1). The drugs were plated at 5 concentrations in 10-fold dilution series. The challenge here was to score the individual drug sensitivities in a patient testing setup, where limited sources of fresh, primary cancer cells are available to quantify selective responses in comparison to control cells from healthy donors. Here, we functionally profiled 22 bone marrow aspirates from 14 AML patients, whereas 4 bone marrow samples from healthy donors tested in the same way were used as controls. In the present study, a total of 5,161 sample-compound pairs were analyzed using the DSS analysis pipeline (Figure 1).
To test its quantitative performance, we first systematically evaluated the predictive power of DSS in terms of its accuracy at differentiating between visually-classified active and inactive compounds across the AML patient and control samples (Figure 2a). Especially the integrated and normalized DSS2 and DSS3 versions systematically improved the sensitivity of the drug efficacy detections at each specificity level, when compared to using the relative IC50 parameter alone (p < 10−5, DeLong's test, DSS vs. relative IC50; Figure 2b). The Activity Area (AA) score also showed comparable sensitivity at the highest specificity levels, but its accuracy significantly decreased after moving beyond the most obvious active cases (p = 6.6 × 10−9, DeLong's test, DSS3 vs. AA), resulting in similar overall performance with IC50 (p = 0.499, DeLong's test, IC50 vs. AA; Figure 2b). This was expected since the AA calculation was developed under more focused settings (Supplementary Fig. 1a). While all the response scores could accurately detect the compounds exhibiting the highest efficacy, the DSS3 proved especially informative for capturing the subtle differences between the drugs showing low or no activity (Supplementary Table 2).
We next evaluated the performance of DSS in terms of how accurately it can cluster drugs in our oncology compound collection according to their known mechanisms of action (MoA). The differential dDSS response profiles across the AML patient and healthy bone marrow control samples were clustered to reveal similarities and differences in selective drug response patterns between the AML patients (Figure 1e). The unsupervised drug clustering reflected closely the classification of the drugs based on their established MoA (Figure 3a). We note that there is no unambiguous one-to-one mapping between generic MoA classes and many polypharmacological compounds, explaining why the response-driven clustering does not perfectly agree with the MoA-based drug classification. For instance, while majority of VEGFR family and ABL tyrosine kinase inhibitors clustered together, nilotinib and tandutinib clustered with mTOR/PI3K inhibitors. However, DSS3 response profiling systematically improved the match to the primary MoA classes, compared with IC50 or AA (p < 5 × 10−4, permutation test; Figure 3b; Supplementary Fig. 2), demonstrating that DSS calculation enables functional grouping of diverse set of compounds in order to predict MoA of uncharacterized drugs.
DSS calculation improves the response scoring resolution in cancer cell models
To evaluate how the DSS performs in more controlled settings, we utilized in vitro profiling data from the published CCLE study, where various cancer cell lines were screened against 24 anticancer compounds3. We first compared the response patterns of PLX4720, a selective RAF family kinase inhibitor, across the wild type BRAF and BRAF-V600E mutated melanoma cell lines; this case study was also used in the original work to demonstrate the operation of the AA score3. The AA and DSS3 calculations provided similar power to detect the selective sensitivity of PLX4720 treatment in the BRAF-V600E mutated cells, compared with the non-mutated BRAF cells (p < 10−15, Wilcoxon rank-sum test; Figure 4a, Supplementary Fig. 3). While these two response scores showed similar distributional patterns in the mutated cells, the DSS3 had lower responses in the WT cells, compared to the AA, after re-scaling these metrics to the same range (p < 10−50, Wilcoxon rank-sum test). The IC50 response parameter was not able to detect the selectivity of PLX4720 to BRAF-V600E mutated cells, rather it scored wild-type cells as more sensitive to PLX4720. These results indicate that the DSS outperforms the conventional activity metrics, such as IC50, and shows comparable selectivity to that of the recently introduced AA measure in this selected cell line case study.
In the second CCLE case example, we compared the distributions of the three activity scores in response to the MEK1/2 kinase inhibitor PD−0325901 in hematopoietic and lymphoid cell lines with or without RAS mutations based on the fact that the MEK1 and MEK2 kinases are key signaling components downstream of RAS oncogenes. Similar to the BRAF-V600E:PLX4720 example, DSS3 and AA were able to detect that the RAS-mutated cell lines as a group were more sensitive to MEK1/2 inhibition compared with non-mutated cells (p = 0.0024 and p = 0.012, respectively, Wilcoxon rank-sum test; Figure 4b). Interestingly, among the most MEK inhibitor sensitive cell lines, there were also several non-RAS mutated cells, indicating that the RAS mutational status is not the sole determinant of the MEK inhibitor sensitivity. Upon closer inspection of these highly sensitive cells (highlighted in green in Figure 4b and Supplementary Table 3), we noticed that they were predominantly AML-derived cell lines, irrespective of their mutational status. Hence, while MEK inhibitors are not magic bullets for treating cancers carrying mutated RAS isoforms in general, a sub-population of AMLs, including those with RAS mutations, appear highly addicted to MEK signaling and might serve as a promising disease cohort to explore MEK inhibitor therapy.
As a third application case, we selected data for a set of 26 breast cancer cell lines from the CCLE resource and studied their differential responses to lapatinib, a clinically approved dual EGFR and ERBB2 (HER2) kinase inhibitor. A subset of four cell lines resulted in significantly higher DSS3 response, compared with the others (p = 0.00013, Wilcoxon rank-sum test), suggesting that these lines are addicted to HER2 signaling (Figure 4c). Such multimodality was not seen in either the IC50 or AA distributions (Figure 4c), whereas the sensitized sub-group was readily detectable by significant positive skewness in the DSS3 distribution (γ = 1.484, p = 0.021, D'Agostino test). All the four cell lines (SK-BR-3, ZR-75-30, AU-565, and BT-474) are known to harbor HER2 amplifications and overexpression. To confirm that these responses were specifically linked to HER2 addiction, we showed that the four lapatinib-responsive cell lines were insensitive to erlotinib (p > 0.75, Wilcoxon rank-sum test), a compound that is known to target specifically EGFR but not HER2 (Supplementary Table 4). Interestingly, in the set of 26 breast cancer cells, there were also HER2-amplified cases, such as HCC1569, which showed relatively low sensitivity to lapatinib (Figure 4c), indicating that HER2 positivity does not necessarily imply HER2 addiction.
DSS calculation improves the identification of drug-sensitive AML patient groups
Finally, we identified a number of examples of translational importance, where positive skewness of the DSS distribution allowed us to distinguish sub-groups of AML patient samples with unique and novel sensitivities to specific drugs. The first such example is ruxolitinib, a recently approved JAK inhibitor for myelofibrosis, which showed increased differential DSS3 response in five AML patient samples (p = 0.00063, Wilcoxon rank-sum test; Figure 5a), but whose selective response was totally missed by the AA or IC50 distributions (Supplementary Fig. 4). Ruxolitinib has previously been explored in patients with relapsed or refractory leukemias, with results showing high heterogeneity in individual response patterns10. It is currently undergoing phase II trials for advanced adult AML patients (clinicaltrials.gov; NCT01251965; NCT00674479), but surprisingly without any molecular or functional biomarkers as inclusion criteria, suggesting that the response rate may end up being low. Strikingly, the DSS-based sample stratification gave us novel insights into the characteristics of those advanced AML cases that are highly responsive to JAK inhibitors ex vivo, which could be promising cases to treat with ruxolitinib in the clinic.
Similarly, we observed that ex vivo drug response to the histone deacetylase (HDAC) inhibitor entinostat resulted in distinctly multimodal DSS3 distribution, where all of the controls and most of the patient samples were clustered into a low-response background group, whereas four of the patient samples formed an distinct outlier group (252_2, 718, 600_2, and 393_3), which showed significantly higher response levels (p = 0.00013, Wilcoxon rank-sum test; Figure 5b; Supplementary Fig. 4). Entinostat is currently undergoing several phase II clinical trials for treatment of various cancers, including AML and myelodysplastic syndrome. Also in this case it is striking that no molecular or phenotypic biomarker inclusion criteria are used in the ongoing trials. Based on our results, entinostat may induce beneficial epigenetic modifications in a specific subgroup of AML patients only, warranting its further testing in more stratified clinical trials. In general, these examples demonstrate that DSS calculation provides a quantitative and highly selective means to identify drug-sensitive subgroups of patient samples that are likely to benefit from a particular drug treatment.
A majority of AML patient samples ex vivo appear to be addicted to kinase signaling5,6. To map the molecular dependencies in the patient cells, we compared the dDSS response profiles with the drug target profiles from a published set of kinase inhibitor specificities11. This allowed us to identify potential kinase-driven signals that the particular patient cells may be addicted to (see Methods). As a case study, we studied two serial samples from the same AML patient (252) before and after treatment with the tyrosine kinase inhibitor dasatinib. At the compound-level, DSS highlighted a reduced sensitivity to a number of kinase inhibitors, such as dasatinib, after the treatment (Figure 6a). At the target-level, the kinase addiction score supported the decreased addiction to the activity of multiple kinase targets (Figure 6b). The kinase addiction network provided an additional view of changes in the target addiction scores before and after the dasatinib treatment (Figure 6c). Such integrated network approach facilitates not only mapping of the key oncologic signals underlying the initial treatment sensitivity, but also following-up and understanding the mechanisms behind the acquired resistance during the disease evolution6.
We have shown that the model-based drug sensitivity quantitation effectively captures and integrates complementary information extracted by IC50, slope and other activity parameters from the complex dose-response relationships. The importance of considering information from multiple response parameters was recently shown in cancer cell line drug testing applications9. We used here AML as the primary disease model since the driving molecular signals underlying AML are still poorly understood, and there is no standardized and effective second line AML treatment, resulting in very poor prognosis for relapsed patients. While next-generation sequencing of clinical AML samples has allowed for extensive cataloging of recurring mutations, these have not yet provided links to clinically actionable therapeutic strategies in most individual cases, perhaps because of our limited understanding of the complex genetic events and extensive clonal heterogeneity that induce and drive an AML6,12. The difficulty of making predictive links between the molecular patterns and drug sensitivity or resistance was also exemplified here in the two AML patient examples (Figure 5). The DSS calculation was implemented and tested in this study to provide a standardized means for functional investigation of druggable vulnerabilities in individual cancer samples ex vivo, even in the absence of genetic or epigenetic profiling information, thereby providing complementary insights into cancer phenotypes and cellular addictions on an individualized basis.
The continuous model estimation makes the DSS calculation robust against many sources of technical variability. For instance, the model fitting enables interpolation of missing values at intermediate concentration levels. Further, while many of the single response parameters, such as IC50, are dependent on the concentration ranges being tested, the summary response metrics, such AA and DSS, provide more comparable results also for compounds tested under different concentration windows. The robustness of the area-based metrics was confirmed in AML cell line models, where AA and DSS response profiles showed improved reproducibility compared to IC50 (Supplementary Fig. 5). Increased inconsistency and problems in extrapolating IC50 levels was also recently noted in the comparison between CCLE and Sanger cell line drug testing data13. Importantly, area-based metrics enable straightforward calculation of the differential responses, relative to that of the control samples, while differential IC50 is not so straightforward to interpret. In many case examples, the waterfall plots of the drug response distributions over a set of patients were relatively uniform when plotted using standard metrics such as IC50. In contrast, especially the DSS3 version, which involved further normalization of AUC by the active dose range, was shown to amplify the differential responses, making the systematic identification of sensitive patient subgroups more straightforward (e.g. Figures 4 and 5).
The previously introduced AA metric provides an approximation of the AUC through a discrete rectangle method (Supplementary Fig. 1a). AA was shown to work well on a smaller collection of 24 compounds, with relatively narrow bioactivity spectra and densely-sampled concentration ranges, carefully centered around an expected IC50 for the primary target of the inhibitors to avoid off-target effects3. However, such discrete approximation may provide sub-optimal response estimates under other settings, especially when broader bioactivity spectra are being tested with a more complex set of compounds. The basic version of DSS (named DSS1) is an AUC measurement with the baseline noise subtracted and therefore conceptually similar to AA (Supplementary Fig. 1b). However, the further normalizations of the DSS calculation make the DSS2 and DSS3 versions different from the AA. In particular, DSS3 captures additional dose-response relationships, and it was shown to outperform AA in the AML case studies, which rely on sparsely-sampled data from limited sources of primary cells (Figure 2 and 3, Supplementary Figs. 2 and 4). An additional advantage of DSS3 in the clinical settings is its reduced correlation with the blast counts (Supplementary Fig. 9), which make samples with varying leukemic blast percentages easier to cross-compare. Further, DSS3 version is able to distinguish toxic response patterns that show activity at the highest dose levels only from the clinically more relevant patterns that show potency over a wider therapeutic window, even if their AUC is similar (see e.g. the entinostat example in Supplementary Fig. 7).
The adequate performance of the DSS was shown here both in controlled cancer cell line models as well as in clinical patient-derived applications. Therefore, we believe it should benefit a range of drug testing applications in vitro and ex vivo. Further, it was shown in our previous work that the observed ex vivo drug responses are predictive of the in vivo treatment response observed in the clinic6. As with any response score, however, the dose-response curves behind the top DSS hits should be visually confirmed before clinical decision making. As a future development, calculation of confidence intervals would provide estimate of the uncertainty of response scores, such as IC50, AUC and DSS, in studies where enough sample material is available for technical replicates or increased number of dose levels sampled for parameter confidence estimation. However, the current DSS implementation was already shown to enable statistical identification of such patient subgroups that are most likely to benefit from a treatment (Figures 4 and 5), whereas DSS profiles across the compounds revealed mechanistic similarities among those drugs showing correlated response patterns (Figure 3). Further, when combined with information on the cellular targets of the most sensitive and selective drugs, one cannot only start identifying pharmacologically targetable oncogenic driver signals (Figure 6), but also to monitor and identify potential mechanisms behind in vivo emerging resistance to the targeted agents6. We expect that this integrated approach will help us to predict next line of more effective treatment strategies, such as multi-targeted combination therapies14, for each individual refractory patient, and will complement the genomic profiling approaches for AML and other cancers.
As a primary clinical evaluation material, we used a set of 22 bone marrow aspirates from 14 mainly relapsed and refractory AML patients, as well as 4 bone marrow samples from healthy donors as controls from our ongoing study6. All the samples were fresh and collected in EDTA treated tubes after informed consent with approval (No. 239/13/03/00/2010, 303/13/03/01/2011), in accordance with the ethical standards of the Helsinki University Central Hospital (HUCH), approved by HUCH Institutional Review Board (Dnro 60/2011). The drug sensitivity and resistance testing (DSRT) was performed as previously described6. Briefly, ex vivo DSRT was run on mononuclear cells isolated from AML patient or healthy bone marrow aspirates using Ficoll density gradient (Ficoll-Paque PREMIUM; GE Healthcare) suspended in Mononuclear Cell Medium (MCM; PromoCell) supplemented with 0.5 μg/mL gentamicin and 2.5 μg/mL amphotericin B. The oncology screening panel used in the present study included 204 compounds covering the approved cancer small molecule pharmacopeia and the active substances of emerging investigational and experimental anticancer compounds, including signal transduction inhibitors targeting major kinase and non-kinase targets (Supplementary Table 1). The compounds were dissolved in DMSO and pre-printed on tissue culture treated 384-well plates (Corning) with an acoustic liquid handling device, Echo 550 (Labcyte Inc.). Each compound was tested in five different concentrations covering a 10,000-fold concentration range (no technical replicates). Prior to addition of the cells, the compounds were dissolved in 5 μL of MCM for 30 min on a plate shaker. Single cell suspension (10,000 cells/well in 20 μL) was transferred to each well with a peristaltic dispenser (MultiDrop Combi; Thermo Scientific). The plates were incubated at 37°C for 72 h, after which the cell viability was measured using CellTiter-Glo luminescent assay (Promega) according to manufacturer's instructions with a Molecular Devices Paradigm plate reader. The response readout was normalized in relation to negative control (DMSO) and positive control (100 μM benzethonium chloride), resulting in relative growth inhibition %. The raw dose-response data were processed in Dotmatics Browser/Studies software (Dotmatics Ltd.), and then subjected to the DSS analysis pipeline (see Supplementary Methods).
Cell line material
As an additional evaluation material, we made use of the set of 479 cell lines screened against 24 anticancer compounds from the Cancer Cell Line Encyclopedia (CCLE) resource3. Dose response for those compounds was measured in dilutions at 8 different concentrations, namely 2.5, 8, 25, 80, 250, 800, 2,530 and 8,000 nM. The medians over the technical replicates of the dose responses were used in the DSS calculation (Supplementary Methods). In the CCLE cell line case studies, we used the Activity Area (AA) values reported in the Supplementary material of the original work3. For comparative evaluations in our AML samples and controls, we implemented the Activity Area calculation according to the descriptions in the Supplementary Material of the original work3, with the help of instructions from one of the authors (personal communication with Dr. Joseph Lehár). The AA calculation is based on discrete summing of the differences between the measured response (relative growth inhibition %) and the reference level (response set to zero) over the eight dose levels (so-called rectangle approximation of the integral function, see Supplemental Fig. 1a). AA has a value of zero, when there is no drug activity and eight for compounds with 100% inhibition across all the eight drug concentrations.
To objectively identify compounds whose response distributions show exceptional positive response, that is, a relatively few highly responsive samples at the right tail of the drug response distribution, we calculated the sample skewness γ of the drugs' empirical response distribution over all the samples under analysis. The one-sided significance p-value of the observed positive skewness was assessed using the D'Agostino15 test in the R-package “moments” (version 0.13, http://cran.r-project.org/package=moments). This enables systematic detection of drug-sensitive patient sub-groups for a given compound, without visually going through all the drug response distributions. When comparing two sets of samples, such as highly responsive patient samples against the remaining samples for those compounds initially identified with positive skewness, we assessed the difference in the response levels between the two pre-defined sample groups with the Wilcoxon rank-sum test. We chose to use the non-parametric test because the response distributions cannot be assumed to be normally distributed.
The predictive accuracy of the DSS, IC50 and AA metrics was assessed in terms of their capability to distinguish the active dose-response curves from the inactive ones using the receiver operating characteristic (ROC) analyses; ROC curves evaluate the relative trade-off between true positive rate (sensitivity) and false positive rate (1 – specificity) of the metric when ordering the dose-response curves according to the increasing value of the response metric16. The overall accuracy of each response metric was summarized using the area under the ROC curve (AUROC) measure; for an ideal metric, AUROC = 1, whereas a random metric obtains an AUROC = 0.5 on average. Statistical significance of an observed AUROC, when compared to random classifier, was assessed using the roc.area function in the R-package “verification”. Statistical significance of an observed AUROC difference between two response metrics was assessed using the “pROC” package with the De Long's test17.
Kinase addiction scoring
To identify the selective kinase targets the individual AML samples may be addicted to, we compared the sample-specific dDSS response with the target profiles of 35 kinase inhibitors overlapping between our compound panel and the kinase inhibitors whose target specificity was biochemically profiled in a recent kinome-wide study11. We designed a kinase inhibition sensitivity score (KISS), which estimates how sensitive the cells are to inhibition to a specific kinase target (or in other words, the addiction to the activity of the given kinase). Formally, for each kinase target k, we calculated KISS by summing the dDSS values over those kinase inhibitors i that selectively target k: Here, the sum is through those nk inhibitors that specifically target the kinase k and whose skewness γ shows significant positive selectivity (p < 0.05, D'Agostino test15). These selective drug response and target profiles were used to define putative kinase addiction pathways for each individual sample, that is, the connected sets of selective kinases that the individual leukemia cells are likely to be addicted to. This is similar to the concept of kinase pathway dependence5. The identified kinase addiction sub-networks for the patient samples were visualized using the automated layout options in the Cytoscape network analysis software18.
Response profile clustering
To reveal similarities and differences in the drug response patterns over the samples, the DSS, IC50 and AA drug response profiles were grouped into functionally similar drug clusters using unsupervised hierarchical clustering technique, Ward's algorithm19. The Spearman's correlation coefficient was used as the similarity function, because the rank-based correlation provided relatively robust and reproducible results between different runs. The evaluation of the clustering solutions was carried out using external cluster evaluation indices, which measure performance by matching the identified clustering solution to a priori information of the drugs. Here, the external benchmarking drug clusters corresponded to the known mode of action (MoA) classes of the drugs, if available (Supplemental Table 1). MoA classes with less than three drugs were excluded, since these present too narrow and potentially unstable drug classes for the cluster validation purposes.
More specifically, we first determined the response score-based drug clusters by cutting branches off the hierarchical clustering dendrogram using the “dynamicTreeCut” library20. The obtained drug partitions were then compared to the MoA drug classes using thee different cluster evaluation indices. The Rand index has a value between 0 and 1, with 0 indicating that the two partitions do not agree on any pair of drugs, and 1 indicating that the drug clusters are exactly the same21. In the Jaccard index, value 1 indicates that one of the partitions lies completely within the other, and value 0 indicates that the partitions have no common drugs. With the Fowlkes–Mallows index, a higher value indicates a higher similarity between the two drug partitions, whereas for two unrelated partitions the index approaches zero as the number of drugs increases22.
Statistical differences in the cluster evaluation indices between the response scores were tested through permutation-based null-distributions. More specifically, a large set of random cluster assignments was simulated by randomly shuffling the drug labels in the observed clustering solutions, separately for DSS, IC50 and AA metrics, while preserving the number of clusters in the original clustering solutions. The random null-model for the differences was obtained by taking pairwise differences in the index values between any two random drug cluster assignments (e.g one for DSS and the other for AA score). The empirical p-value was calculated by counting the number of the pairwise random permutations having greater or equal index difference value when compared to the observed difference, divided by the number of random permutations (here: 10,000).
The authors thank the patients in donating their samples for our research; Jani Saarela, Anna Lehto, Ida Lindenschmidt and Laura Turunen at the High Throughput Biomedicine Unit of Institute for Molecular Medicine Finland (FIMM) for technical assistance; Biocenter Finland research infrastructures for technical and infrastructural support; Joseph Lehár at Novartis Institute for Biomedical Research and Nicolas Stransky at Broad Institute of Harvard and MIT for their help with the activity area calculation; the CCLE consortium for making the comprehensive cancer cell line data publicly available; Leena Karhinen at FIMM for her help in the annotation of the breast cancer cell lines; and Julia Casado at Aalto University School of Science for her technical assistance with the drug response data fitting. This work was supported by the Finnish Funding Agency for Technology and Innovation (Tekes), the European Regional Development Fund (ERDF), the Finnish Cancer Societies, the Sigrid Jusélius Foundation, the Jane and Aatos Erkko Foundation, Biocenter Finland and the Academy of Finland.