Biomarkers of immune checkpoint inhibitor response and toxicity: Challenges and opportunities.

Immune checkpoint inhibitors have transformed cancer therapy, but their optimal use is still constrained by lack of response and toxicity. Biomarkers of response may facilitate drug development by allowing appropriate therapy selection and focusing clinical trial enrollment. However, aside from PD-L1 staining in a subset of tumors and rarely mismatch repair deficiency, no biomarkers are routinely used in the clinic. In addition, severe toxicities may cause severe morbidity, therapy discontinuation, and even death. Here, we review the state of the field with a focus on our research in therapeutic biomarkers and toxicities from immune checkpoint inhibitors.

Therapeutic Responses to Combination Nivolumab and Temozolomide as Salvage Therapy for Metastatic Melanoma: A Case Series.

The management of metastatic melanoma patients that fail multiple lines of systemic therapy remains a significant challenge. There is limited literature regarding combination of anti-PD-1 and temozolomide, or of other chemotherapy agents, in melanoma. Here, we present a series of 3 patients with metastatic melanoma and their responses to nivolumab and temozolomide combination therapy after progression on several local/regional therapies, combination immune checkpoint inhibitors, and/or targeted therapies. The novel combinatory strategy led to remarkable responses in all 3 patients shortly after initiating treatment with tumor remission and symptomatic improvement. The first patient has had ongoing response 15 months after initiating treatment, although he has since discontinued temozolomide due to intolerance. The remaining 2 patients show ongoing response after 4 months, with good tolerability. This case series suggests that nivolumab and temozolomide may be a promising option in the setting of advanced melanoma refractory to standard treatments, and warrants further investigation in larger series.

Prognostic Risk Stratification and End-of-Life Care Outcomes in Patients With Metastatic Melanoma Treated With Immune Checkpoint Inhibitors.

INTRODUCTION: The emergence of immune checkpoint inhibitors (ICIs) has improved survival outcomes in patients with metastatic melanoma, while potentially increasing the use of systemic therapy near the end of life (EOL). Yet, less is known on how to facilitate treatment decision making and identify patients who might benefit from early palliative care comanagement. MATERIALS AND METHODS: We determined baseline clinical and laboratory factors that are associated with poor prognosis for patients with advanced melanoma treated with ICIs. We subsequently identified prognostic subgroups to evaluate association with EOL outcomes and determine if EOL care varied across prognostic strata. RESULTS: Our cohort included 398 patients with metastatic melanoma treated with ICIs. Factors associated with overall survival (OS) included: lactate dehydrogenase, neutrophil/lymphocyte ratio, performance status, prior therapies, liver metastases, and lung metastases. Patients were stratified by risk of death using risk scores developed from multivariable analyses. A total of 205 patients died: 45/133 (34%) low-risk, 63/133 (47%) medium-risk, and 97/132 (73%) of high-risk patients. Among those who died, higher risk patients were more likely to receive ICIs within 14, 30, and 90 days of death. We found no association between risk group and hospice referrals or location of death. CONCLUSION: Patients with metastatic melanoma at highest risk of death as defined by our model were more likely than lower-risk patients to receive ICIs near the EOL. Prognostic risk stratification may guide early palliative care interventions to appropriately utilize ICIs and optimize EOL care.

Cross-Sectional Study of Psoriasis, Atopic Dermatitis, Rosacea, and Alopecia Areata Suggests Association With Cardiovascular Diseases.

BACKGROUND: A growing body of evidence suggests that several inflammatory skin diseases (ISDs) are associated with systemic inflammation and cardiovascular disease (CVDs). METHODS: We used the TriNetX analytics platform to conduct a retrospective, cross-sectional, single-center study in the Mount Sinai Health System network. Cases (all patients ≥18 years of age with a diagnosis of 1 of the 4 ISDs studied) were compared with matched controls (no history of any of these ISDs) to evaluate odds ratios for being diagnosed with CVD. RESULTS: We identified a total of 70,090 patients with ISDs, including 35,160 patients with atopic dermatitis, 19,490 with psoriasis, 12,470 with rosacea, and 2,970 with alopecia areata, and 70,090 propensity score-matched controls without any of these ISDs. Patients with atopic dermatitis and psoriasis had significantly increased odds of all CVD diagnoses analyzed compared to controls (P<0.001 for all comparisons). Patients with rosacea had significantly increased odds of being diagnosed with all diseases of the circulatory system (P<0.001), hypertensive diseases (P<0.001), cerebrovascular diseases (P=0.037), and arterial disease (P<0.001) compared to controls. Patients with alopecia areata had increased odds for all diseases of the circulatory system (P<0.001), hypertensive diseases (P<0.001), and arterial disease (P<0.001). The prevalence of patients with elevated C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) levels was significantly greater in all ISD groups compared to controls. CONCLUSION: This study identified significant associations between ISDs and several CVD diagnoses. Furthermore, CRP and ESR were elevated in all ISD groups compared to controls. Pagan AD, Jung S, Caldas S, et al. Cross-sectional study of psoriasis, atopic dermatitis, rosacea, and alopecia areata suggests association with cardiovascular diseases. J Drugs Dermatol. 2023;22(6):576-581. doi:10.36849/JDD.7424.

Clinical and genomic correlates of imatinib response in melanomas with KIT alterations.

BACKGROUND: Imatinib is an active agent for some patients with melanoma harbouring c-KIT alterations. However, the genetic and clinical features that correlate with imatinib sensitivity are not well-defined. METHODS: We retrospectively evaluated 38 KIT-altered melanoma patients from five medical centres who received imatinib, and pooled data from prospective studies of imatinib in 92 KIT-altered melanoma patients. Baseline patient and disease characteristics, and clinical outcomes were assessed. RESULTS: In the pooled analysis (N = 130), alterations in exons 11/13 had the highest response rates (38% and 33%); L576P (N = 23) and K642E (N = 12) mutations had ORR of 52% and 42%, respectively. ORR was 38% (mucosal), 25% (acral), and 8% (unknown-primary). PFS appeared longer in exon 11/13 vs. exon 17 alterations (median 4.3 and 4.5 vs. 1.1 months; p = 0.19), with similar superiority in OS (median 19.7 and 15.4 vs. 12.1 months; p = 0.20). By histology, median PFS was 4.5 months (mucosal), 2.7 (acral), and 5.0 (unknown-primary) [p = 0.36]. Median OS was 18.0 months (mucosal), 21.8 (acral), 11.5 (unknown-primary) [p = 0.26]. In multivariate analyses, mucosal melanoma was associated with higher PFS and exon 17 mutations were associated with reduced PFS. CONCLUSION: This multicenter study highlights KIT-alterations sensitive to imatinib and augments evidence for imatinib in subsets of KIT-altered melanoma.

Management of Acral and Mucosal Melanoma: Medical Oncology Perspective.

Acral and mucosal melanomas (MM) are rare subtypes of melanoma that are biologically and clinically distinct from cutaneous melanoma. Despite the progress in the treatment of cutaneous melanomas with the development of targeted and immune therapies, the therapeutic options for these less common subtypes remain limited. Difficulties in early diagnosis, the aggressive nature of the disease, and the frequently occult sites of origin have also contributed to the poor prognosis associated with acral and MM, with substantially worse long-term prognosis. The rarity of these subtypes has posed significant barriers to better understanding their biological features and investigating novel therapies. Consequently, establishing standardized treatment guidelines has been a challenge. In this review, we provide a brief overview of the current knowledge regarding acral and MM, focusing on their epidemiology, genetic backgrounds, and unique clinical characteristics. Further discussion centers around the management of primary and advanced disease and the role of emerging targeted and immune therapies for these subtypes, specifically focusing on issues relevant to medical oncologists.

Recurrence of Hypophysitis After Immune Checkpoint Inhibitor Rechallenge.

Immune checkpoint inhibitor (ICI)-induced hypophysitis is an immune-mediated pituitary inflammation that tends to cause long-term pituitary deficiency. Management of ICI-induced hypophysitis includes corticosteroids for acute inflammation and long-term hormone replacement due to irreversible pituitary cell damage. We report a case of recurrent hypophysitis following ICI rechallenge for metastatic melanoma. A 33-year-old woman with recurrent metastatic melanoma with adrenal, pelvic, and inguinal metastases developed recurrent hypophysitis during treatment with ipilimumab and nivolumab which recurred with rechallenge >5 years later. In both cases, headache was the most notable symptom and brain MRI showed pituitary enlargement and edema without evidence of metastases. Central adrenal insufficiency and symptoms caused by mass effect were treated with acute high-dose corticosteroids and long-term replacement corticosteroids. Based on recurrence and failure of symptomatic treatment with continued steroid treatment, ICI was discontinued. This case illustrates that hypophysitis may recur with ICI rechallenge, challenging traditional assumptions that chronic, irreversible irAEs are unlikely to recur or flare. The regenerative potential of pituitary cells after ICI-induced damage or additional damage to previously unaffected cells may be more conceivable than previously realized. Additional research on the potential for recurrent ICI-induced endocrinopathies are needed.

Prognostic significance of FDG-PET/CT in determining upfront autologous stem cell transplantation for the treatment of peripheral T cell lymphomas.

This observational study aimed to evaluate the prognostic significance of interim and final 18F-fluorodeoxyglucose positron emission tomography-computed tomography (PET/CT) responses to upfront autologous stem cell transplantation (ASCT) in patients with peripheral T cell lymphomas (PTCLs). A total of 118 patients, from two independent institutions, with newly diagnosed PTCLs were enrolled, and 96 of them were evaluated. PET/CT was assessed at diagnosis, and during and after the primary treatment. Clinical outcomes of interim and final PET/CT were compared between transplanted and non-transplanted patients. The responses of PET/CT were assessed based on visual analysis using the Deauville five-point scale (5-PS). Clinicopathological features of transplanted patients (n = 37) were similar to those of non-transplanted patients (n = 59). After a median follow-up of 60.8 months, only final PET/CT response based on 5-PS was the independent prognostic factor of survival outcome (P < 0.001; HR 8.215; 95% C.I. 2.97-22.72) in multivariate analysis. Interim PET/CT response did not have a differential potential for predicting progression-free survival (PFS). In 59 patients, with score 1 or 2 in final PET/CT, the PFS rate was not significantly different between transplanted and non-transplanted patients (P = 0.970). Moreover, among the 37 patients with final PET/CT response score of 3-4, the PFS rate was equally poor in both transplanted and non-transplanted patients (P = 0.178). Final PET/CT assessment, based on 5-PS, was an important prognostic parameter for primary treatment of PTCLs, regardless of upfront ASCT. Interim PET/CT response could not be an indicator to determine the requirement for upfront ASCT.

18F-FDG PET/CT is useful for determining survival outcomes of patients with multiple myeloma classified as stage II and III with the Revised International Staging System.

PURPOSE: This study evaluated the prognostic role of 18F-FDG PET/CT at baseline in patients with newly diagnosed multiple myeloa (MM) and evaluated the prognostic relevance of 18F-FDG PET/CT for each stage according to the Revised International Staging System (R-ISS). METHOD: We retrospectively analyzed the records of 167 patients with newly diagnosed MM. 18F-FDG PET/CT was performed prior to induction therapy in patients with newly diagnosed MM. RESULTS: In the total cohort, the presence of more than three hypermetabolic focal lesions (FLs) or extramedullary disease (EMD) on baseline PET/CT was associated with significantly inferior progression-free survival (PFS) and overall survival (OS) than other patients. Because most patients (91%) with EMD had more than three FLs, PET/CT positivity was defined as the presence of more than three FLs or the presence of EMD. In multivariate analyses, PET/CT positivity was an independent predictor of PFS and OS in all patients. Fifty-five patients (46.1%) with R-ISS II were PET/CT-positive at baseline and had significantly shorter PFS and OS. PET/CT positivity was also correlated with poor PFS and OS in patients with R-ISS III. CONCLUSION: 18F-FDG PET/CT was an independent predictor of survival outcomes in patients with newly diagnosed MM. In addition, performing 18F- FDG PET/CT at diagnosis may be useful for determining the survival outcomes of MM patients with R-ISS II and III.

Optimal chemo-mobilization for the collection of peripheral blood stem cells in patients with multiple myeloma.

BACKGROUND: For successful autologous stem cell transplantation, the collection of a sufficient number of hematopoietic stem cells after induction therapy is essential for transplant candidates with multiple myeloma (MM). METHODS: In this study, we compared the efficacy and safety of stem cell mobilization using cyclophosphamide (CY; 3.0 g/m2 on day 1) or etoposide (VP-16; 375 mg/m2 on days 1 and 2) in patients with MM. Granulocyte-colony stimulating factor (G-CSF, 10 µg/kg/day, subcutaneously) was administered from the onset of neutropenia to the final day of collection. RESULTS: Sixty-five patients were mobilized with a combination of CY and G-CSF, and 63 were mobilized with a combination of VP-16 and G-CSF. All patients were mobilized within 7 months of beginning frontline treatment. The median number of CD34+ cells collected was significantly higher in the VP-16 mobilization group than in the CY mobilization group (27.6 ×  106 CD34+/kg vs. 9.6 × 106 CD34+/kg, P <  0.001). The rate of mobilization failure, defined as < 2.0 × 106 CD34+/kg collected in three apheresis procedures, was lower in the VP-16 group than in the CY group (1.6% vs. 10.8%, P = 0.062). Severe infections during the mobilization period were more frequent in the CY group than in the VP-16 group (18.5% vs. 7.9%, P = 0.117). CONCLUSION: In conclusion, an intermediate dose of VP-16 with G-CSF appears to be an effective and tolerable chemo-mobilization method compared to CY and G-CSF, particularly in cases where use plerixafor in MM is difficult.

Pralatrexate in Combination with Bortezomib for Relapsed or Refractory Peripheral T Cell Lymphoma in 5 Elderly Patients.

Peripheral T cell lymphoma (PTCL) is a heterogeneous group of aggressive lymphomas with poor prognosis. Elderly (age ≥ 65years) patients generally have impaired bone marrow function, altered drug metabolism, comorbidities, and poor functional status. Thus, treatment of elderly patients with relapsed or refractory PTCL remains a challenge for clinicians. A recent study disclosed that pralatrexate has a synergistic effect in combination with bortezomib. Weekly pralatrexate and bortezomib were administered intravenously for 3 weeks in a 4-week cycle. Of 5 patients, one achieved complete response after 4 cycles which has lasted 12 months until now. Another patient attained partial response after 2 cycles. Only 1 patient experienced grade 3 thrombocytopenia and neutropenia. Two patients suffered from grade 3 mucositis. Combination therapy with pralatrexate and bortezomib may be used as a salvage therapy for relapsed or refractory PTCL in the elderly with a favorable safety profile.

Benefit, recurrence pattern, and toxicity to adjuvant anti-PD-1 monotherapy varies by ethnicity and melanoma subtype: An international multicenter cohort study.

Background: Anti-Program-Death-1 (PD-1) is a standard adjuvant therapy for patients with resected melanoma. We hypothesized that there are discrepancies in survival, recurrence pattern and toxicity to adjuvant PD-1 between different ethnicities and melanoma subtypes. Objective: We performed a multicenter cohort study incorporating 6 independent institutions in Australia, China, Japan, and the United States. The primary outcomes were recurrence free survival (RFS) and overall survival (OS). Secondary outcomes were disease recurrence patterns and toxicities. Results: In total 534 patients were included. East-Asian/Hispanic/African reported significantly poorer RFS/OS. Nonacral cutaneous or melanoma of unknown primary reported the best RFS/OS, followed by acral, and mucosal was the poorest. Within the nonacral cutaneous or melanoma of unknown primary subtypes, East-Asian/Hispanic/African reported significantly poorer RFS/OS than Caucasian. In the multivariate analysis incorporating ethnicity/melanoma-subtype/age/sex/stage/lactate dehydrogenase/BRAF (v-Raf murine sarcoma viral oncogene homolog B)-mutation/adjuvant radiotherapy, East-Asian/Hispanic/African had independently significantly poorer outcomes (RFS: HR, 1.71; 95% CI, 1.19-2.44 and OS: HR, 2.34; 95% CI, 1.39-3.95), as was mucosal subtype (RFS: HR, 3.25; 95% CI, 2.04-5.17 and OS: HR, 3.20; 95% CI, 1.68-6.08). Mucosal melanoma was an independent risk factor for distant metastasis, especially liver metastasis. East-Asian/Hispanic/African had significantly lower incidence of gastrointestinal/musculoskeletal/respiratory/other-rare-type-toxicities; but higher incidences of liver toxicities. Limitations: A retrospective study. Conclusions: Ethnicity and melanoma subtype are associated with survival and recurrence pattern in melanoma patients treated with adjuvant anti-PD-1. Toxicity profile differs by ethnicity and may require a precision toxicity surveillance strategy.

Alopecia Areata: Current Treatments and New Directions.

Alopecia areata is an autoimmune hair loss disease that is non-scarring and is characterized by chronic inflammation at the hair follicle level. Clinically, patients' presentation varies from patchy, circumscribed scalp involvement to total body and scalp hair loss. Current management is guided by the degree of scalp and body involvement, with topical and intralesional steroid injections as primarily first-line for mild cases and broad immunosuppressants as the mainstay for more severe cases. Until recently, the limited number of blinded, randomized, placebo-controlled clinical trials for this disease had made establishing an evidence-based treatment paradigm challenging. However, growing insights into the pathogenesis of alopecia areata through blood and tissue analysis of human lesions have identified several promising targets for therapy. T-helper (Th) 1/interferon skewing has traditionally been described as the driver of disease; however, recent investigations suggest activation of additional immune mediators, including the Th2 pathway, interleukin (IL)-9, IL-23, and IL-32, as contributors to alopecia areata pathogenesis. The landscape of alopecia areata treatment has the potential to be transformed, as several novel targeted drugs are currently undergoing clinical trials. Given the recent US FDA approval of baricitinib and ritlecitinib, Janus kinase (JAK) inhibitors are a promising drug class for treating severe alopecia areata cases. This article will review the efficacy, safety, and tolerability of current treatments for alopecia areata, and will provide an overview of the emerging therapies that are leading the revolution in the management of this challenging disease.

Dabrafenib plus trametinib versus anti-PD-1 monotherapy as adjuvant therapy in BRAF V600-mutant stage III melanoma after definitive surgery: a multicenter, retrospective cohort study.

Background: Both dabrafenib/trametinib (D/T) and anti-PD-1 monotherapy (PD-1) are approved adjuvant therapies for patients with stage III BRAF V600-mutant melanoma. However, there is still a lack of head-to-head comparative data. We aimed to describe efficacy and toxicity outcomes for these two standard therapies across melanoma centers. Methods: This multicenter, retrospective cohort study was conducted in 15 melanoma centers in Australia, China, Germany, Italy, Japan, UK, and US. We included adult patients with resected stage III BRAF V600-mutant melanoma who received either adjuvant D/T or PD-1 between Jul 2015 and Oct 2022. The primary endpoint was relapse-free survival (RFS). Secondary endpoints included overall survival (OS), recurrence pattern and toxicity. Findings: We included 598 patients with stage III BRAF V600-mutant melanoma who received either adjuvant D/T (n = 393 [66%]) or PD-1 (n = 205 [34%]) post definitive surgery between Jul 2015 and Oct 2022. At a median follow-up of 33 months (IQR 21-43), the median RFS was 51.0 months (95% CI 41.0-not reached [NR]) in the D/T group, significantly longer than PD-1 (44.8 months [95% CI 28.5-NR]) (univariate: HR 0.66, 95% CI 0.50-0.87, P = 0.003; multivariate: HR 0.58, 95% CI 0.39-0.86, P = 0.007), with comparable OS with PD-1 (multivariate, HR 0.90, 95% CI 0.48-1.70, P = 0.75). Similar findings were observed using a restricted-mean-survival-time model. Among those who experienced recurrence, the proportion of distant metastases was higher in the D/T cohort. D/T had a higher incidence of treatment modification due to adverse events (AEs) than PD-1, but fewer persistent AEs. Interpretation: In patients with stage III BRAF V600-mutant melanoma post definitive surgery, D/T yielded better RFS than PD-1, with higher transient but lower persistent toxicity, and comparable OS. D/T seems to provide a better outcome compared with PD-1, but a longer follow-up and ideally a large prospective trial are needed. Funding: Dr. Xue Bai was supported by the Beijing Hospitals Authority Youth Programme (QMS20211101) for her efforts devoted to this study. Dr. Keith T. Flaherty was funded by Adelson Medical Research Foundation for the efforts devoted to this study.

Challenging Dermatologic Considerations Associated with Immune Checkpoint Inhibitors.

Immune checkpoint inhibitors have emerged as a new paradigm in oncologic care for many malignancies. However, nonspecific immune activation has led to "collateral damage" in the form of immune-related adverse events, with skin being a commonly affected organ. Cutaneous immune-related adverse events include a wide spectrum of clinical presentations and challenging considerations, often necessitating dermatology referral to support diagnosis and management, particularly for atypical presentations or more severe, cutaneous immune-related adverse events that may require specialized dermatologic evaluations including biopsy and histopathology. Close collaborations between oncologists and dermatologists may optimize clinical decision making in the following challenging management settings: non-steroidal therapies for corticosteroid-refractory, cutaneous immune-related adverse events, immune checkpoint inhibitor rechallenge, balancing cutaneous immune-related adverse events and treatments, and immune checkpoint inhibitors in patients with pre-existing autoimmune disease, skin conditions, and organ transplants. These complex clinical decisions that often lack rigorous data should be made in close collaboration with dermatologists to minimize unnecessary morbidity and mortality. This article provides a review of approaches to challenging dermatologic considerations associated with immune checkpoint inhibitor therapies.

Allogeneic transplant can abrogate the risk of relapse in the patients of first remission acute myeloid leukemia with detectable measurable residual disease by next-generation sequencing.

In patients with acute myeloid leukemia (AML) consolidation treatment options are between allogeneic hematopoietic stem cell transplantation (HCT) and chemotherapy, based on disease risk at the time of initial presentation and age. Measurable residual disease (MRD) following induction chemotherapy could be incorporated as a useful parameter for treatment decisions. The present study evaluated treatment outcomes according to the next-generation sequencing (NGS)-based MRD status and the type of consolidation therapy in patients with normal karyotype (NK)-AML. By sequencing 278 paired samples collected at diagnosis and first remission (CR1), we identified 361 mutations in 124 patients at diagnosis and tracked these at CR1. After excluding mutations associated with age-related clonal hematopoiesis, 82 mutations in 50 of the 124 patients (40.3%) were detected at CR1. Survival benefit was observed in favor of allogeneic HCT over chemotherapy consolidation in the MRDpos subgroup with respect to overall survival (HR 0.294, p = 0.003), relapse-free survival (HR 0.376, p = 0.015) and cumulative incidence of relapse (HR 0.279, p = 0.004) in multivariate analysis, but not in the MRDneg subgroup. In summary, these data support allogeneic HCT in NK-AML patients with detectable MRD by NGS in CR1. Randomized clinical trials will be required to confirm this observation.

Remission clone in acute myeloid leukemia shows growth advantage after chemotherapy but is distinct from leukemic clone.

In a previously published case study of acute myeloid leukemia, we tracked the dynamics of somatic mutations over 9 years. Interestingly, we observed a group of mutations that expanded during remission, which we named the "remission clone." To determine the nature of the remission clones, we performed flow cytometry-based cell sorting followed by ultradeep sequencing. The remission clone repeatedly expanded after chemotherapeutic cycles and was suppressed during relapse in the myeloid lineage (multipotent hematopoietic stem, progenitor, and myeloid cells). On the other hand, the remission clone was consistently observed in lymphoid lineages (B and T cells) regardless of the disease state. When transfected into the HEK-293 cell line, the NR2C2(A93V) mutant exhibited a growth advantage (all p values < 0.05). The results indicate that the remission clone seems to be another form of clonal hematopoiesis, but without a clear association with leukemia. As the remission clone is present in both myeloid and lymphoid lineages, it likely originates from ancestral hematopoietic cell lineages. More importantly, the remission clone is distinct from the leukemic clone; therefore, mutations expanded during remission require special interpretation when performing next-generation sequencing-based measurable residual disease assessment.

Microtubule minus-end aster organization is driven by processive HSET-tubulin clusters.

Higher-order structures of the microtubule (MT) cytoskeleton are comprised of two architectures: bundles and asters. Although both architectures are critical for cellular function, the molecular pathways that drive aster formation are poorly understood. Here, we study aster formation by human minus-end-directed kinesin-14 (HSET/KIFC1). We show that HSET is incapable of forming asters from preformed, nongrowing MTs, but rapidly forms MT asters in the presence of soluble (non-MT) tubulin. HSET binds soluble (non-MT) tubulin via its N-terminal tail domain to form heterogeneous HSET-tubulin clusters containing multiple motors. Cluster formation induces motor processivity and rescues the formation of asters from nongrowing MTs. We then show that excess soluble (non-MT) tubulin stimulates aster formation in HeLa cells overexpressing HSET during mitosis. We propose a model where HSET can toggle between MT bundle and aster formation in a manner governed by the availability of soluble (non-MT) tubulin.