Analyzing tyrosine kinase activity in head and neck cancer by functional kinomics: Identification of hyperactivated Src family kinases as prognostic markers and potential targets.

Signal transduction via protein kinases is of central importance in cancer biology and treatment. However, the clinical success of kinase inhibitors is often hampered by a lack of robust predictive biomarkers, which is also caused by the discrepancy between kinase expression and activity. Therefore, there is a need for functional tests to identify aberrantly activated kinases in individual patients. Here we present a systematic analysis of the tyrosine kinases in head and neck cancer using such a test-functional kinome profiling. We detected increased tyrosine kinase activity in tumors compared with their corresponding normal tissue. Moreover, we identified members of the family of Src kinases (Src family kinases [SFK]) to be aberrantly activated in the majority of the tumors, which was confirmed by additional methods. We could also show that SFK hyperphosphorylation is associated with poor prognosis, while inhibition of SFK impaired cell proliferation, especially in cells with hyperactive SFK. In summary, functional kinome profiling identified SFK to be frequently hyperactivated in head and neck squamous cell carcinoma. SFK may therefore be potential therapeutic targets. These results furthermore demonstrate how functional tests help to increase our understanding of cancer biology and support the expansion of precision oncology.

Comparative effectiveness trial of transoral head and neck surgery followed by adjuvant radio(chemo)therapy versus primary radiochemotherapy for oropharyngeal cancer (TopROC).

BACKGROUND: For loco-regionally advanced, but transorally resectable oropharyngeal cancer (OPSCC), the current standard of care includes surgical resection and risk-adapted adjuvant (chemo) radiotherapy, or definite chemoradiation with or without salvage surgery. While transoral surgery for OPSCC has increased over the last decade for example in the United States due to transoral robotic surgery, this treatment approach has a long history in Germany. In contrast to Anglo-Saxon countries, transoral surgical approaches have been used frequently in Germany to treat patients with oro-, hypopharyngeal and laryngeal cancer. Transoral laser microsurgery (TLM) has had a long tradition since its introduction in the early 70s. To date, the different therapeutic approaches to transorally resectable OPSCC have not been directly compared to each other in a randomized trial concerning disease control and survival. The goal of this study is to compare initial transoral surgery to definitive chemoradiation for resectable OPSCC, especially with regards to local and regional control. METHODS: TopROC is a prospective, two-arm, open label, multicenter, randomized, and controlled comparative effectiveness study. Eligible patients are ≥18 years old with treatment-naïve, histologically proven OPSCC (T1, N2a-c, M0; T2, N1-2c, M0; T3, N0-2c, M0 UICC vers. 7) which are amenable to transoral resection. Two hundred eighty patients will be randomly assigned (1:1) to surgical treatment (arm A) or chemoradiation (arm B). Standard of care treatment will be performed according to daily routine practice. Arm A consists of transoral surgical resection with neck dissection followed by risk-adapted adjuvant therapy. Patients treated in arm B receive standard chemoradiation, residual tumor may be subject to salvage surgery. Follow-up visits for 3 years are planned. Primary endpoint is time to local or locoregional failure (LRF). Secondary endpoints include overall and disease free survival, toxicity, and patient reported outcomes. Approximately 20 centers will be involved in Germany. This trial is supported by the German Cancer Aid and accompanied by a scientific support program. DISCUSSION: This study will shed light on an urgently-needed randomized comparison of the strategy of primary chemoradiation vs. primary surgical approach. As a comparative effectiveness trial, it is designed to provide data based on two established regimens in daily clinical routine. TRIAL REGISTRATION: NCT03691441 Registered 1 October 2018 - Retrospectively registered.

Survival analysis of 287 oropharyngeal squamous cell carcinoma patients in a single institution: a retrospective comparison of two consecutive time intervals with surgical and conservative treatment approaches.

This study is a retrospective analysis of clinico-pathological data to investigate survival rates of patients with oropharyngeal squamous cell carcinoma (OPSCC) treated with different modalities in a single academic head and neck cancer center in different time intervals. Altogether, 287 patients with OPSCC were included in this comparison. Patients were analysed during two different treatment periods: Group 1 included patients treated mainly with primary surgery ± adjuvant radio(chemo)therapy between 2002 and 2007, while Group 2 included patients treated with organ/function-preservation protocols if indicated. Main outcome measures were overall survival (OS) and recurrence-free survival (RFS). Between 2002 and 2007, early-stage OPSCC showed a 5-year OS of 75% compared to that of 86% between 2008 and 2013. Locally advanced OPSCC showed a 5-year OS of 66% between 2002 and 2007 compared to that of 74% between 2008 and 2013. RFS in early-stage OPSCC was 48% between 2002 and 2007 in contrast to that of 77% between 2008 and 2013. With locally advanced OPSCC, RFS was 55% between 2002 and 2007 compared to that of 56% between 2008 and 2013. These differences were statistically not significant. The OS and RFS remained generally unchanged over the analysed time period. There was no significant difference in the outcomes with regards to HPV status and to their treatment modality.

Perspectives in chemosensitivity and chemoresistance assays and their implementation in head and neck cancer.

Therapeutic options for patients with head and neck squamous cell carcinoma include surgery, radiation and chemotherapy. The latter plays a major role in the treatment selection of recurrent, metastatic or therapy resistant tumours, these being some of the major challenges in head and neck oncology. For these patients, chemosensitivity and chemoresistance assays would be paramount to identify their individual therapy options. In this review, seven common assays will be described and discussed in the context of several studies. Further, a new assay will also be presented, currently being validated in a European Union funded project. Comparisons will be drawn to evaluate the sensitivity and specificity of these assays in identifying individual treatment options, and their potential implementation in head and neck malignancies will be discussed. There is an unmet demand for the development of ex vivo diagnostic tools that may predict response in head and neck cancer on the way towards an individualized treatment for these patients.

Prognostic factors for survival of patients with newly diagnosed chronic GVHD according to NIH criteria.

Chronic graft versus host disease (cGvHD) is the most common cause of late morbidity and mortality after allogeneic hematopoietic stem cell transplantation (HSCT). We retrospectively evaluated the impact of NIH classification on outcome of patients at our center. Primary endpoint was overall survival at 5 years. Two hundred one patients with cGVHD according to NIH were included. Platelets <100,000/µl on day of diagnosis of cGvHD (HR 2.97, 95 % CI 1.7-5.3, p < 0.001), female donor (HR 1.78, 95 % CI 1.0-3.2, p = 0.05), and reduced intensity conditioning (HR 1.95, 95 % CI 1.0-3.8, p = 0.05) impacted overall survival. Non-relapse mortality (NRM) was higher for patients with low vs. high platelets: 26 % (95 % CI 14-40) vs. 6 % (95 % CI 2-10), p < 0.001, and tended to be higher for female vs. male donor: 14 % (95 % CI 7-23) vs. 7 % (95 % CI 3-13), p = 0.08. Relapse tended to be higher for recipients of reduced intensity conditioning (RIC) vs. myeloablative conditioning (MAC): 33 % (95 % CI 23-43) vs. 20 % (95 % CI 10-31), p = 0.06. After excluding patients with myeloma and lymphoma, IgG serum levels at diagnosis of cGvHD of 122 patients were correlated with survival. IgG levels above normal were associated with worse 2-year overall survival (OS), p = 0.04, compared to normal or low IgG levels. Platelet count at diagnosis remains the most valid prognostic factor for survival of patients with cGvHD even in the era of NIH grading. High IgG level at diagnosis of cGVHD represents a potential negative prognostic parameter that deserves further investigation.

Serum albumin level predicts survival of patients with gastrointestinal acute graft-versus-host disease after allogeneic stem cell transplantation.

In a retrospective single-centre study, we analysed the prognostic impact of factors identifiable at initial diagnosis of acute GVHD (aGVHD). We retrospectively analysed 495 adult patients of whom 308 (62 %) developed acute GVHD (I-IV) and were included in further analysis. Gut aGVHD was diagnosed in 163/308 cases (53 %). Conditioning was myeloablative conditioning (MAC) in 123 (39.9 %) and reduced intensity (RIC) in 185 (60.1 %) patients. Median serum albumin level at diagnosis of aGVHD was 34 g/l, which was used as cut-off for low vs. normal albumin levels. In patients with gut aGVHD, low albumin level at the time of diagnosis of aGVHD was associated with poorer overall survival (OS) which was 52 vs. 67 % at 1 year and 40 vs. 61 % at 3 years, p = 0.015. In patients with only skin aGVHD, 1- and 3-year OS of patients with low vs. normal albumin levels were 72 vs. 72 % and 59 vs. 57 %, respectively, p = 0.69. In multivariate analysis of patients with gut aGVHD, low serum albumin level ≤34 g/l (relative risk (RR) 2.13, p = 0.003), gut aGVHD grades 3-4 (RR 2.70, p = 0.001), RIC (RR 1.84, p = 0.024), matched unrelated donor (RR 1.86, p = 0.18) and mismatched unrelated donor (RR 2.76, p = 0.03) retained negative impact on OS. Subgroup analysis revealed that impact of albumin was restricted to patients with gut aGVHD after RIC. Low serum albumin levels are associated with poorer OS in patients with gut but not skin aGVHD after RIC but not MAC allogeneic stem cell transplantation.

SHIP1 Is Present but Strongly Downregulated in T-ALL, and after Restoration Suppresses Leukemia Growth in a T-ALL Xenotransplantation Mouse Model.

Acute lymphoblastic leukemia (ALL) is the most common cause of cancer-related death in children. Despite significantly increased chances of cure, especially for high-risk ALL patients, it still represents a poor prognosis for a substantial fraction of patients. Misregulated proteins in central switching points of the cellular signaling pathways represent potentially important therapeutic targets. Recently, the inositol phosphatase SHIP1 (SH2-containing inositol 5-phosphatase) has been considered as a tumor suppressor in leukemia. SHIP1 serves as an important negative regulator of the PI3K/AKT signaling pathway, which is frequently constitutively activated in primary T-ALL. In contrast to other reports, we show for the first time that SHIP1 has not been lost in T-ALL cells, but is strongly downregulated. Reduced expression of SHIP1 leads to an increased activation of the PI3K/AKT signaling pathway. SHIP1-mRNA expression is frequently reduced in primary T-ALL samples, which is recapitulated by the decrease in SHIP1 expression at the protein level in seven out of eight available T-ALL patient samples. In addition, we investigated the change in the activity profile of tyrosine and serine/threonine kinases after the restoration of SHIP1 expression in Jurkat T-ALL cells. The tyrosine kinase receptor subfamilies of NTRK and PDGFR, which are upregulated in T-ALL subgroups with low SHIP1 expression, are significantly disabled after SHIP1 reconstitution. Lentiviral-mediated reconstitution of SHIP1 expression in Jurkat cells points to a decreased cellular proliferation upon transplantation into NSG mice in comparison to the control cohort. Together, our findings will help to elucidate the complex network of cell signaling proteins, further support a functional role for SHIP1 as tumor suppressor in T-ALL and, much more importantly, show that full-length SHIP1 is expressed in T-ALL samples.

Src family kinase targeting in head and neck tumor cells using SU6656, PP2 and dasatinib.

BACKGROUND: We have recently shown a frequent upregulation of Src-family kinases (SFK) in head and neck squamous cell carcinoma (HNSCC). Here we tested, if SFK targeting is effective especially in HNSCC cells with upregulated SFK signaling. METHODS: The impact of SFK inhibitors SU6656, PP2 and dasatinib on three HNSCC cell lines with different SFK activity levels was analyzed using proliferation and colony formation assays, Western blot and functional kinomics. RESULTS: Proliferation was blocked by all inhibitors in a micro-molar range. With respect to cell kill, dasatinib was most effective, while SU6656 showed moderate and PP2 minor effects. Cellular signaling was affected differently, with PP2 having no effect on SFK signaling while dasatinib probably has non-SFK specific effects. Only SU6656 showed clear SFK specific effects on signaling. CONCLUSION: The results demonstrate potential benefit of SFK inhibition in HNSCC but they also highlight challenges due to non-specificities of the different drugs.

Impaired DNA double-strand break repair and effective radiosensitization of HPV-negative HNSCC cell lines through combined inhibition of PARP and Wee1.

Objectives: In head and neck squamous cell carcinoma (HNSCC), tumors negative for Human Papillomavirus (HPV) remain a difficult to treat entity and the morbidity of current multimodal treatment is high. Radiotherapy in combination with molecular targeting could represent suitable, less toxic treatment options especially for cisplatin ineligible patients. Therefore, we tested dual targeting of PARP and the intra-S/G2 checkpoint through Wee1 inhibition for its radiosensitizing capacity in radioresistant HPV-negative HNSCC cells. Materials and methods: Three radioresistant HPV-negative cell lines (HSC4, SAS, UT-SCC-60a) were treated with olaparib, adavosertib and ionizing irradiation. The impact on cell cycle, G2 arrest and replication stress was assessed through flow cytometry after DAPI, phospho-histone H3 and γH2AX staining. Long term cell survival after treatment was determined through colony formation assay and DNA double-strand break (DSB) levels were assessed through quantification of nuclear 53BP1 foci in cell lines and patient-derived HPV± tumor slice cultures. Results: Wee1 and dual targeting induced replication stress but failed to effectively inhibit radiation-induced G2 cell cycle arrest. Single as well as combined inhibition increased radiation sensitivity and residual DSB levels, with the largest effects induced through dual targeting. Dual targeting also enhanced residual DSB levels in patient-derived slice cultures from HPV-negative but not HPV+ HNSCC (5/7 vs. 1/6). Conclusion: We conclude that the combined inhibition of PARP and Wee1 results in enhanced residual DNA damage levels after irradiation and effectively sensitizes radioresistant HPV-negative HNSCC cells. Ex vivo tumor slice cultures may predict the response of individual patients with HPV-negative HNSCC to this dual targeting approach.

Patient derived ex vivo tissue slice cultures demonstrate a profound DNA double-strand break repair defect in HPV-positive oropharyngeal head and neck cancer.

BACKGROUND: HPV-positive head and neck squamous cell carcinoma of the oropharynx (OPSCC) are more sensitive towards radiation than HPV-negative OPSCC. Two main theories exist regarding the underlying mechanism. Stronger lymphocyte infiltration points to an enhanced immunogenicity, whereas data from HPV-positive HNSCC cell lines suggest an enhanced cellular radiosensitivity based on a defect in DNA double-strand break (DSB) repair. The critical limitation of the latter theory is that the evidence was largely derived from a small number of established HPV-positive HNSCC cell lines. METHODS AND MATERIALS: Fresh patient-derived OPSCC samples were cut in 400 µm sections and cultured on cell culture inserts. Slice cultures were irradiated, in part combined with ATM inhibition, and fixed and frozen after 2 and 24 h. DSBs were analyzed by quantification of 53BP1 foci in nuclei co-stained with the SCC marker p63 via immunofluorescence microscopy. RESULTS: Ex vivo OPSCC tumor slice cultures maintained stable oxygenation and proliferation characteristics for at least 3 days. Areas of p63-positivity in immunofluorescence microscopy matched histologically confirmed tumor cell areas in serial sections, indicating the suitability of p63 as a tumor cell marker. p63-positive nuclei in HPV-positive OPSCC tissues (n = 14) showed profoundly elevated numbers of residual radiation-induced DSBs as compared to those from HPV-negative OPSCC (n = 12) (3 Gy: on average 4.9 vs. 1.2 foci per nucleus; p < 0.0001). Within the HPV-positive subgroup, samples derived from patients with a smoking history of less than 10 pack years demonstrated higher residual DSBs as compared to those derived from patients with 10 or more pack years (3 Gy: on average 6.5 vs. 3.2 foci per nucleus; p = 0.0105). Additional ATM inhibition resulted in a substantial increase in residual foci in all 4 HPV-negative samples tested but strikingly only in 2 out of 11 HPV-positive samples. CONCLUSIONS: In summary, our data provide robust, cell line-independent experimental evidence for an intrinsic DSB repair deficiency in HPV-positive OPSCC, strongly suggesting a meaningful contribution to the enhanced clinical radiosensitivity. The reduced effectiveness of ATM inhibition indicates a defect in the ATM-orchestrated DNA damage response. Lower numbers of residual 53BP1 nuclear foci in the ex vivo assay may identify HPV-positive patients with effective DSB repair who should potentially be excluded from de-intensification approaches.

Increased replication stress and R-loop accumulation in EGFRvIII-expressing glioblastoma present new therapeutic opportunities.

Background: The oncogene epidermal growth factor receptor variant III (EGFRvIII) is expressed in approximately one-third of all glioblastomas (GBMs). So far it is not clear if EGFRvIII expression induces replication stress in GBM cells, which might serve as a therapeutical target. Methods: Isogenetic EGFRvIII- and EGFRvIII+ cell lines with endogenous EGFRvIII expression were used. Markers of oncogenic and replication stress such as γH2AX, RPA, 53BP1, ATR, and CHK1 were analyzed using western blot, immunofluorescence, and flow cytometry. The DNA fiber assay was performed to analyze replication, transcription was measured by incorporation of EU, and genomic instability was investigated by micronuclei and CGH-Array analysis. Immunohistochemistry staining was used to detect replication stress markers and R-loops in human GBM samples. Results: EGFRvIII+ cells exhibit an activated replication stress response, increased spontaneous DNA damage, elevated levels of single-stranded DNA, and reduced DNA replication velocity, which are all indicative characteristics of replication stress. Furthermore, we show here that EGFRvIII expression is linked to increased genomic instability. EGFRvIII-expressing cells display elevated RNA synthesis and R-loop formation, which could also be confirmed in EGFRvIII-positive GBM patient samples. Targeting replication stress by irinotecan resulted in increased sensitivity of EGFRvIII+ cells. Conclusion: This study demonstrates that EGFRvIII expression is associated with increased replication stress, R-loop accumulation, and genomic instability. This might contribute to intratumoral heterogeneity but may also be exploited for individualized therapy approaches.

Rational surgical neck management in total laryngectomy for advanced stage laryngeal squamous cell carcinomas.

PURPOSE: Controversies exist in regard to surgical neck management in total laryngectomies (TL). International guidelines do not sufficiently discriminate neck sides and sublevels, or minimal neck-dissection nodal yield (NY). METHODS: Thirty-seven consecutive primary TL cases from 2009 to 2019 were retrospectively analyzed in terms of local tumor growth using a previously established imaging scheme, metastatic neck involvement, and NY impact on survival. RESULTS: There was no case of level IIB involvement on any side. For type A and B tumor midline involvement, no positive contralateral lymph nodes were found. Craniocaudal tumor extension correlated with contralateral neck involvement (OR: 1.098, p = 0.0493) and showed increased involvement when extending 33 mm (p = 0.0134). Using a bilateral NY of ≥ 24 for 5-year overall survival (OS) and ≥ 26 for 5-year disease-free survival (DFS) gave significantly increased rate advantages of 64 and 56%, respectively (both p < 0.0001). CONCLUSIONS: This work sheds light on regional metastatic distribution pattern and its influence on TL cases. An NY of n ≥ 26 can be considered a desirable benchmark for bilateral selective neck dissections as it leads to improved OS and DFS. Therefore, an omission of distinct neck levels cannot be promoted at this time.

Kinomic comparison of snap frozen and ex vivo-cultured head and neck tumors.

OBJECTIVES: The use of primary tumor tissue in experimental and pre-clinical cancer research is becoming increasingly important. Especially the use of tissue slice cultures of tumor specimen, so called ex vivo cultures or tumor explants, promises functional analysis under approximate physiological conditions. This includes screening and testing of targeted therapeutics directed against deregulated protein kinases. However, it is unclear if ex vivo cultures indeed represent the in situ situation especially with respect to very sensitive and transient molecular processes such as kinase dependent signaling. We now asked here, if and to what extent ex vivo culturing affects kinase activity. MATERIALS AND METHODS: We analyzed the activity of protein tyrosine kinases (PTK) using functional kinome profiling of either snap frozen or ex vivo-cultured tumor tissue samples of head and neck cancer patients. RESULTS: Although we observed a quantitative decline in overall kinase activity after 24 h or 48 h of ex vivo cultivation, we most importantly noticed that the signaling characteristics were conserved in most samples; approximately two thirds of all ex vivo-cultured samples displayed a signaling pattern which was qualitatively comparable to the parental tumor. We could also demonstrate kinase inhibition by treatment of ex vivo slice cultures with the multi-kinase inhibitor staurosporine, although higher concentrations were needed compared to cell cultures. CONCLUSION: We here demonstrate that the tyrosine kinase dependent signaling is conserved under exvivo culturing conditions in the majority of samples, which highlights the power of this method in experimental and pre-clinical cancer research.

EGFRvIII upregulates DNA mismatch repair resulting in increased temozolomide sensitivity of MGMT promoter methylated glioblastoma.

The oncogene epidermal growth factor receptor variant III (EGFRvIII) is frequently expressed in glioblastomas (GBM) but its impact on therapy response is still under controversial debate. Here we wanted to test if EGFRvIII influences the sensitivity towards the alkylating agent temozolomide (TMZ). Therefore, we retrospectively analyzed the survival of 336 GBM patients, demonstrating that under standard treatment, which includes TMZ, EGFRvIII expression is associated with prolonged survival, but only in patients with O6-methylguanine-DNA methyltransferase (MGMT) promoter methylated tumors. Using isogenic GBM cell lines with endogenous EGFRvIII expression we could demonstrate that EGFRvIII increases TMZ sensitivity and results in enhanced numbers of DNA double-strand breaks and a pronounced S/G2-phase arrest after TMZ treatment. We observed a higher expression of DNA mismatch repair (MMR) proteins in EGFRvIII+ cells and patient tumor samples, which was most pronounced for MSH2 and MSH6. EGFRvIII-specific knockdown reduced MMR protein expression thereby increasing TMZ resistance. Subsequent functional kinome profiling revealed an increased activation of p38- and ERK1/2-dependent signaling in EGFRvIII expressing cells, which regulates MMR protein expression downstream of EGFRvIII. In summary, our results demonstrate that the oncoprotein EGFRvIII sensitizes a fraction of GBM to current standard of care treatment through the upregulation of DNA MMR.

Receptor tyrosine kinase MET as potential target of multi-kinase inhibitor and radiosensitizer sorafenib in HNSCC.

BACKGROUND: The multi-kinase inhibitor sorafenib displays antitumoral effects in head and neck squamous cell carcinoma (HNSCC); however, the targeted kinases are unknown. Here we aimed to identify those kinases to determine the mechanism of sorafenib-mediated effects and establish candidate biomarkers for patient stratification. METHODS: The effects of sorafenib and MET inhibitors crizotinib and SU11274 were analyzed using a slide-based antibody array, Western blotting, proliferation, and survival assays. X-rays were used for irradiations. RESULTS: Sorafenib inhibited auto-phosphorylation of epidermal growth factor receptor and MET, which has not been described previously. MET expression in HNSCC cells was not always associated with activity/phosphorylation. Furthermore, sorafenib-dependent cell kill and radiosensitization was not associated with MET level. Although MET inhibitors blocked proliferation, they caused only mild cytotoxicity and no radiosensitization. CONCLUSION: We identified MET as a new potential target of sorafenib. However, MET inhibition is not the cause for sorafenib-mediated cytotoxicity or radiosensitization.

Analyzing expression and phosphorylation of the EGF receptor in HNSCC.

Overexpression of the epidermal growth factor receptor (EGFR) in head and neck squamous cell carcinomas (HNSCC) is considered to cause increased EGFR activity, which adds to tumorigenicity and therapy resistance. Since it is still unclear, whether EGFR expression is indeed associated with increased activity in HNSCC, we analyzed the relationship between EGFR expression and auto-phosphorylation as a surrogate marker for activity. We used a tissue micro array, fresh frozen HNSCC tumor and corresponding normal tissue samples and a large panel of HNSCC cell lines. While we observed substantial overexpression only in approximately 20% of HNSCC, we also observed strong discrepancies between EGFR protein expression and auto-phosphorylation in HNSCC cell lines as well as in tumor specimens using Western blot and SH2-profiling; for the majority of HNSCC EGFR expression therefore seems not to be correlated with EGFR auto-phosphorylation. Blocking of EGFR activity by cetuximab and erlotinib points to increased EGFR activity in samples with increased basal auto-phosphorylation. However, we could also identify cells with low basal phosphorylation but relevant EGFR activity. In summary, our data demonstrate that EGFR expression and activity are not well correlated. Therefore EGFR positivity is no reliable surrogate marker for EGFR activity, arguing the need for alternative biomarkers or functional predictive tests.

Axitinib: A Photoswitchable Approved Tyrosine Kinase Inhibitor.

The goal of photopharmacology is to develop photoswitchable enzyme modulators as tunable (pro-)drugs that can be spatially and temporally controlled by light. In this context, the tyrosine kinase inhibitor axitinib, which contains a photosensitive stilbene-like moiety that allows for E/Z isomerization, is of interest. Axitinib is an approved drug that targets the vascular endothelial growth factor receptor 2 (VEGFR2) and is licensed for second-line therapy of renal cell carcinoma. The photoinduced E/Z isomerization of axitinib has been investigated to explore if its inhibitory effect can be turned "on" and "off", as triggered by light. Under controlled light conditions, (Z)-axitinib is 43 times less active than that of the E isomer in an VEGFR2 assay. Furthermore, it was proven that kinase activity in human umbilical vein cells (HUVECs) was decreased by (E)-axitinib, but only weakly affected by (Z)-axitinib. By irradiating (Z)-axitinib in vitro with UV light (λ=385 nm), it is possible to switch it almost quantitatively into the E isomer and to completely restore the biological activity of (E)-axitinib. However, switching the biological activity off from (E)- to (Z)-axitinib was not possible in aqueous solution due to a competing irreversible [2+2]-photocycloaddition, which yielded a biologically inactive axitinib dimer.