Somatic gene mutation patterns and burden influence outcomes with enasidenib in relapsed/refractory IDH2-mutated AML

Limited treatment options are available for patients with relapsed/refractory acute myeloid leukemia (R/R AML). We recently reported results from the phase 3 IDHENTIFY trial (NCT02577406) showing improved response rates and event-free survival with enasidenib monotherapy compared with conventional care regimens (CCR) in heavily pretreated, older patients with late-stage R/R AML bearing IDH2 mutations. Here we investigated the prognostic impact of mutational burden and different co-mutation patterns at study entry within the predominant IDH2 variant subclasses, IDH2-R140 and IDH2-R172. The prognostic relevance of these variants is well documented in newly diagnosed AML, but data are lacking in R/R AML. In this large R/R AML patient cohort, targeted next-generation sequencing at baseline (screening) revealed distinct co-mutation patterns and mutational burden between subgroups bearing different IDH2 variants: variant IDH2-R140 was associated with greater mutational burden and was enriched predominantly with poor-risk mutations, including FLT3, RUNX1, and NRAS, while variant IDH2-R172 was associated with lower mutational burden and was preferentially co-mutated with DNMT3A. In multivariable analyses, RAS and RTK pathway mutations were significantly associated with decreased overall survival, after adjusting for treatment arm, IDH2 variant, and mutational burden. Importantly, enasidenib-mediated survival benefit was more pronounced in patients with IDH2-R172 variants.

Survival outcomes in patients with acute myeloid leukaemia who received subsequent therapy for relapse in QUAZAR AML-001

In the phase 3 QUAZAR AML-001 trial (NCT01757535) of patients with acute myeloid leukaemia (AML) in remission following intensive chemotherapy (IC) and ineligible for haematopoietic stem cell transplant (HSCT), oral azacitidine (Oral-AZA) maintenance significantly prolonged overall survival (OS) versus placebo. The impact of subsequent treatment following maintenance has not been evaluated. In this post hoc analysis, OS was estimated for patients who received subsequent AML therapy, and by regimen received (IC or lower-intensity therapy). First subsequent therapy (FST) was administered after treatment discontinuation in 134/238 Oral-AZA and 173/234 placebo patients. OS from randomization in patients who received FST after Oral-AZA versus placebo was 17.8 versus 12.9 months (HR: 0.82 [95% CI: 0.64-1.04], median follow-up: 56.7 months); OS from FST was similar between arms. Among patients who received injectable hypomethylating agents as FST, median OS was 8.2 versus 4.9 months in the Oral-AZA versus placebo groups (HR: 0.66 [95% CI: 0.41-1.06]). Forty-eight patients (16/238 Oral-AZA, 32/234 placebo) received HSCT following treatment discontinuation, including six Oral-AZA patients still in first remission; Oral-AZA OS benefit persisted when censoring these patients. Oral-AZA maintenance can prolong AML remission duration without negatively impacting survival outcomes after salvage therapies.

Oral azacitidine modulates the bone marrow microenvironment in patients with acute myeloid leukaemia in remission: A subanalysis from the QUAZAR AML-001 trial

Oral azacitidine (Oral-AZA) maintenance therapy improved relapse-free (RFS) and overall survival (OS) significantly versus placebo for AML patients in remission after intensive chemotherapy (IC) in the phase 3 QUAZAR AML-001 study. Immune profiling was performed on the bone marrow (BM) at remission and on-treatment in a subset of patients with the aim of identifying prognostic immune features and evaluating associations of on-treatment immune effects by Oral-AZA with clinical outcomes. Post-IC, increased levels of lymphocytes, monocytes, T cells and CD34 + CD117+ BM cells were prognostically favourable for RFS. CD3+ T-cell counts were significantly prognostic for RFS in both treatment arms. At baseline, high expression of the PD-L1 checkpoint marker was identified on a subset of CD34 + CD117+ BM cells; many of which were PD-L2+. High co-expression of T-cell exhaustion markers PD-1 and TIM-3 was associated with inferior outcomes. Oral-AZA augmented T-cell numbers during early treatment, increased CD4+:CD8+ ratios and reversed T-cell exhaustion. Unsupervised clustering analysis identified two patient subsets defined by T-cell content and expression of T-cell exhaustion markers that were enriched for MRD negativity. These results indicate that Oral-AZA modulates T-cell activity in the maintenance setting of AML, and these immune-mediated responses are associated with clinical outcomes.

Prognostic impact of NPM1 and FLT3 mutations in patients with AML in first remission treated with oral azacitidine

The randomized, placebo-controlled, phase 3 QUAZAR AML-001 trial (ClinicalTrials.gov identifier: NCT01757535) evaluated oral azacitidine (Oral-AZA) in patients with acute myeloid leukemia (AML) in first remission after intensive chemotherapy (IC) who were not candidates for hematopoietic stem cell transplantation. Eligible patients were randomized 1:1 to Oral-AZA 300 mg or placebo for 14 days per 28-day cycle. We evaluated relapse-free survival (RFS) and overall survival (OS) in patient subgroups defined by NPM1 and FLT3 mutational status at AML diagnosis and whether survival outcomes in these subgroups were influenced by presence of post-IC measurable residual disease (MRD). Gene mutations at diagnosis were collected from patient case report forms; MRD was determined centrally by multiparameter flow cytometry. Overall, 469 of 472 randomized patients (99.4%) had available mutational data; 137 patients (29.2%) had NPM1 mutations (NPM1mut), 66 patients (14.1%) had FLT3 mutations (FLT3mut; with internal tandem duplications [ITD], tyrosine kinase domain mutations [TKDmut], or both), and 30 patients (6.4%) had NPM1mut and FLT3-ITD at diagnosis. Among patients with NPM1mut, OS and RFS were improved with Oral-AZA by 37% (hazard ratio [HR], 0.63; 95% confidence interval [CI], 0.41-0.98) and 45% (HR, 0.55; 95% CI, 0.35-0.84), respectively, vs placebo. Median OS was improved numerically with Oral-AZA among patients with NPM1mut whether without MRD (48.6 months vs 31.4 months with placebo) or with MRD (46.1 months vs 10.0 months with placebo) post-IC. Among patients with FLT3mut, Oral-AZA improved OS and RFS by 37% (HR, 0.63; 95% CI, 0.35-1.12) and 49% (HR, 0.51; 95% CI, 0.27-0.95), respectively, vs placebo. Median OS with Oral-AZA vs placebo was 28.2 months vs 16.2 months, respectively, for patients with FLT3mut and without MRD and 24.0 months vs 8.0 months for patients with FLT3mut and MRD. In multivariate analyses, Oral-AZA significantly improved survival independent of NPM1 or FLT3 mutational status, cytogenetic risk, or post-IC MRD status.

Overall survival by IDH2 mutant allele (R140 or R172) in patients with late-stage mutant-IDH2 relapsed or refractory acute myeloid leukemia treated with enasidenib or conventional care regimens in the phase 3 IDHENTIFY trial

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Background: IDH2 gene mutations (m IDH2) occur in up to ̃20% of patients (pts) with acute myeloid leukemia (AML), most commonly as R140Q (in ̃75% of cases) or R172K (̃25%) point mutations. The functional effects and prognostic relevance of m IDH2-R140 and m IDH2-R172 can vary (Papaemmanuil 2016). In the randomized, phase 3 IDHENTIFY trial, enasidenib (ENA), an oral mIDH2 inhibitor, did not significantly improve overall survival (OS) vs conventional care regimens (CCR) as salvage treatment (Tx) for older pts with m IDH2 relapsed/refractory (R/R) AML in ITT analysis, but a trend for improved OS with ENA was detected in pts with IDH2-R172 mutations. We further investigated OS and correlative biomarkers in IDHENTIFY pt subgroups defined by m IDH2 variant (R140/R172). Methods: This open-label trial (NCT02577406) enrolled pts ≥ 60 years of age who had received 2 or 3 prior AML Tx. Pts were preselected to a CCR (azacitidine, intermediate- or low-dose Ara-C, or supportive care), and were then randomized 1:1 to ENA 100 mg/d or CCR in 28d cycles. Co-occurring gene mutations were identified by targeted next-generation sequencing (37-gene panel) of bone marrow mononuclear cell (BMMC) DNA. Total 2-HG levels were determined by LC/MS. Results: Of 319 pts enrolled, 88 pts (28%; 43 ENA, 45 CCR) had m IDH2-R172 and 229 (72%; 115 ENA, 114 CCR) had m IDH2-R140. Median baseline (BL) 2-HG levels were similar between Tx arms and m IDH2 variant subgroups, as were IDH2 variant allele frequencies. Pts with m IDH2-R172 had fewer median BL co-mutations (4 [range 2–8]) than did pts with m IDH2-R140 (5 [1–11]) ( P < 0.0001). The most frequently co-occurring mutations were SRSF2 and RUNX1 in the R140 cohort (59% each) and DNMT3A in the R172 cohort (57%). Compared with the R172 cohort, the R140 group was enriched with SRSF2, FLT3 (-ITD/-TKD), NPM1, RUNX1, and JAK2 mutations, whereas DNMT3A and TP53 mutations were more common in the R172 group. In Cox multivariate analysis including m IDH2 variant (R140/R172), DNMT3A mutation status, and number of gene mutations at BL, m IDH2-R172 was significantly ( P = 0.04) correlated with improved OS (vs. R140) in the ENA arm, whereas the number of BL gene mutations was significantly ( P < 0.01) associated with OS in the CCR arm. Median OS in the R172 subgroup was 14.6 mo with ENA vs 7.8 mo with CCR (HR, 0.59 [95%CI 0.35-0.98]; P = 0.039) and 1-yr survival rates were 62% and 30%, respectively. In m IDH2-R140 pts, median OS was 5.7 mo in both Tx arms (0.93 [0.70-1.24]; P = 0.61), and 1-year survival rates were 29% and 25% with ENA and CCR, respectively. Conclusions: Mutational burden and co-mutational profiles differed between pts with m IDH2-R140 and m IDH2-R172 R/R AML. ENA improved survival outcomes for pts with IDH2-R172 mutations, with median OS and 1-year survival rate approximately double those in the CCR arm. Clinical trial information: NCT02577406.

Abstract 505: Oral azacitidine modulates the immune microenvironment in acute myeloid leukemia (AML) patients in remission: A subanalysis from the QUAZAR AML-001 Trial

BACKGROUND The immunologic effects of maintenance therapy in patients (pts) with AML in remission are not well-characterized but of high clinical interest, as rapid recovery of bone marrow (BM) after intensive chemotherapy (IC) may help delay relapse. Post IC, immunological interactions in the BM microenvironment present several immunosuppressive mechanisms. PD-L1 is commonly overexpressed on AML blasts, which is associated with worse prognosis. Oral azacitidine (Oral-AZA [CC-486]) is a hypomethylating agent recently approved in the US for pts with AML in complete remission (CR) or CR with incomplete hematologic recovery (CRi). To better understand the effects of Oral-AZA on immune cells, checkpoint expression of PD-L1/2 on AML blasts and normal myeloid progenitors (MPs), and the kinetics of T cell recovery and activation/exhaustion (eg, PD1, TIM3) were assessed.

METHODS Biomarker-evaluable pts aged ≥ 55 years with AML were randomized 1:1 to Oral-AZA 300 mg (n=56) or placebo (PBO, n=52) post IC within 4 months of achieving CR. Flow cytometry evaluations of BM aspirates were performed at screening (ie, baseline [BL]), every 3 cycles until cycle 24 and every 6 cycles thereafter to cycle 36, or as clinically indicated. Correlative analyses of baseline immune parameters with median (med) relapse-free survival (RFS) were computed using Kaplan-Meier methods.

RESULTS In the biomarker-evaluable pts, PD-L1 and PD-L2 expression at BL were higher on AML blasts (med intensity 7.1 and 2.9) than normal MPs (0.7 and 1.6). Most AML blasts were PD-L2+ (79%), whereas only 1.9% were PD-L1+. When stratified by the med, higher BL CD3 T cell numbers (as a % of total BM ) were associated with favorable RFS in both Tx arms (Oral-AZA: ≥ med, 562 days[d] and &lt; med, 235d [P = .0308]; PBO: ≥ med, 325d and &lt; med, 155d [P = .0391]). At cycle 3, pts in the Oral-AZA arm had a 1.7-fold increase in CD3 T cells from BL (PBO, 1.1; P = .0450), suggesting Oral-AZA can promote immunologic recovery during early Tx cycles. There was an inverse correlation between T cell exhaustion marker phenotypes (PD1/TIM3+) with CD4 (r = -.5967; P &lt; .0001) and CD8 (r = -.2484; P = .0095) T cell numbers. An increase in RFS was seen in the PBO arm with lower PD1/TIM3+ CD4 numbers (&lt; med, 429d; ≥ med, 155d; P = .0037), with a nominal increase observed in the Oral-AZA arm (&lt; med, 428d; ≥ med, 303d; P = .6764). In a subset of pts, Oral-AZA appeared to suppress CD4 T cell exhaustion (PD1/TIM3+) compared with PBO.

CONCLUSIONS Pts in CR/CRi post-IC have a unique immune profile defined by high expression of PD-L1 on a subset of blasts and a high % of PD-L2+ blasts. A higher BL CD3 T cell count after IC in BM was prognostic. Additionally, Oral-AZA appears to contribute to an increase in T cells while also suppressing exhaustion, potentially promoting T cell signaling that could activate functional immune-mediated responses against residual leukemic cells.

Citation Format: Daniel L. Menezes, Wendy L. See, Alberto Risueno, Jianglin Ma, Ignazia La Torre, Barry Skikne, CL Beach, Keshava Kumar, Anjan Thakurta. Oral azacitidine modulates the immune microenvironment in acute myeloid leukemia (AML) patients in remission: A subanalysis from the QUAZAR AML-001 Trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 505.

Combination of azacitidine and enasidenib enhances leukemic cell differentiation and cooperatively hypomethylates DNA

Azacitidine and enasidenib are two therapies available for treatment of acute myelogenous leukemia (AML), and the mechanisms of action of these drugs involve alteration of aberrant DNA methylation. We hypothesized that a combination of these agents could have interactive effects on DNA methylation and enhance differentiation in mIDH2 cells. Combination treatment enhanced cellular differentiation in TF-1 cells overexpressing IDJ2R140Q through increased hemoglobinization and increased hemoglobin γ RNA expression compared with the effects of single agents. Furthermore, in primary AML samples (IDH2R140Q or R172K), combination treatment reduced CD34+ cells and increased CD15+ cells to a greater extent than attained with single agents. To explore the mechanism of enhanced differentiation with combination treatment, the TF-1 epigenome was analyzed by profiling 5-hydroxymethylcytosine (5hmC) and 5-methylcytosine (5mC) DNA methylation changes. Enasidenib treatment alone increased 5hmC, consistent with reactivation of ten-eleven-translocation (TET) enzyme activity. Compared with treatment with azacitidine alone, combination treatment reduced 5mC levels at greater numbers of sites and these loci were significantly enriched in regions with increased 5hMC (25.8% vs. 7.4%). Results are consistent with a model in which enasidenib-mediated reactivation of ten-eleven-translocation enzymes cooperates with azacitidine-mediated inhibition of DNA methyltransferase enzymes, leading to greater reductions in DNA methylation and enhanced erythroid differentiation.

Enasidenib in patients with mutant IDH2 myelodysplastic syndromes: a phase 1 subgroup analysis of the multicentre, AG221-C-001 trial

BACKGROUND

Mutations in isocitrate dehydrogenase-2 (IDH2) occur in around 5% of patients with myelodysplastic syndromes. Neomorphic activity of mutant IDH2 proteins results in hypermethylation of DNA and histones, leading to blocked haemopoietic differentiation. Enasidenib, an inhibitor of mutated IDH2 proteins, induces responses in patients with IDH2-mutated, relapsed or refractory acute myeloid leukaemia. We aimed to establish the clinical outcomes of enasidenib monotherapy in a subgroup of patients with myelodysplastic syndromes harbouring mutations in IDH2 from the AG221-C-001 trial.

METHODS

The multicentre, open-label, phase 1-2 AG221-C-001 trial enrolled patients with advanced haematological malignancies (2008 WHO criteria) harbouring an IDH2 mutation. The present study is a subgroup analysis of patients with IDH2-mutated myelodysplastic syndromes in the phase 1 dose-escalation and expansion portions of the trial. Patients with myelodysplastic syndromes were aged 18 years or older with an ECOG performance status score of 2 or lower, and were relapsed or refractory to, or ineligible for, standard treatments. Patients received oral doses of enasidenib at 60-300 mg per day in repeated 28-day treatment cycles. In this subgroup analysis, we focused on the safety and activity of enasidenib as main outcomes. Overall response rate, duration of response, and overall and event-free survival analyses were by intention-to-treat. Safety was assessed in all participants who received at least one dose of study drug in terms of treatment-emergent adverse events. The AG221-C-001 trial is registered on ClinicalTrials.gov, NCT01915498, status ongoing but closed to recruitment.

FINDINGS

17 patients with myelodysplastic syndromes harbouring an IDH2 mutation (median age, 67·0 years [IQR 60·5-73·0]) were enrolled between Feb 18, 2014, and Sept 1, 2015. At data cutoff (Oct 1, 2018), after a median follow-up of 11·0 months (IQR 6·8-23·0), all patients had discontinued enasidenib, with a median of 3 treatment cycles (2-15) for all patients (five [29%] received ≥12 cycles). At entry, three (18%) patients had relapsed after allogeneic stem-cell transplants, 13 (76%) had previously received therapy with hypomethylating agents, and ten (59%) had received at least two previous therapies. No dose-limiting toxicities were reported. The most common treatment-emergent adverse events were diarrhoea and nausea (in nine [53%] patients each). Most common grade 3-4 treatment-emergent adverse events were indirect hyperbilirubinaemia (in six [35%] patients), pneumonia (in five [29%] patients), and thrombocytopaenia (in four [24%] patients). Serious treatment-emergent adverse events in more than one patient were pneumonia (in five [29% patients); tumor lysis syndrome (in three [18%] patients); and sepsis, atrial flutter, indirect hyperbilirubinaemia, cerebral hemorrhage, and mental status change (in two [12%] patients each). No treatment-related deaths occurred. An overall response was achieved in 9 patients (53% [95% CI 28-77]), with a median duration of response of 9·2 months (95% CI 1·0-not reached). Six (46%) of 13 patients previously treated with hypomethylating agents responded. Median overall survival was 16·9 months (95% CI 1·5-32·3), and median event-free survival was 11·0 months (1·5-16·7).

INTERPRETATION

Enasidenib is generally well tolerated and can induce responses in patients with mutant IDH2 myelodysplastic syndromes, including in those who have had previous therapy with hypomethylating agents. Testing for IDH2 mutations in myelodysplastic syndromes is essential for identifying patients who might benefit from enasidenib therapy, including those patients in whom conventional treatments have been unsuccessful.

FUNDING

Celgene and Agios Pharmaceuticals.

PKM2 uses control of HuR localization to regulate p27 and cell cycle progression in human glioblastoma cells

The M2 isoform of pyruvate kinase (PK) is upregulated in most cancers including glioblastoma. Although PKM2 has been reported to use dual kinase activities to regulate cell growth, it also interacts with phosphotyrosine (pY)-containing peptides independently of its kinase activity. The potential for PKM2 to use the binding of pY-containing proteins to control tumor growth has not been fully examined. We here describe a novel mechanism by which PKM2 interacts in the nucleus with the RNA binding protein HuR to regulate HuR sub-cellular localization, p27 levels, cell cycle progression and glioma cell growth. Suppression of PKM2 in U87, T98G and LN319 glioma cells resulted in increased p27 levels, defects in entry into mitosis, increased centrosome number, and decreased cell growth. These effects could be reversed by shRNA targeting p27. The increased levels of p27 in PKM2 knock-down cells were caused by a loss of the nuclear interaction between PKM2 and HuR, and a subsequent cytoplasmic re-distribution of HuR, which in turn led to increased cap-independent p27 mRNA translation. Consistent with these results, the alterations in p27 mRNA translation, cell cycle progression and cell growth caused by PKM2 suppression could be reversed in vitro and in vivo by suppression of HuR or p27 levels, or by introduction of forms of PKM2 that could bind pY, regardless of their kinase activity. These results define a novel mechanism by which PKM2 regulates glioma cell growth, and also define a novel set of potential therapeutic targets along the PKM2-HuR-p27 pathway.

Mutant IDH1-driven cellular transformation increases RAD51-mediated homologous recombination and temozolomide resistance

Isocitrate dehydrogenase 1 (IDH1) mutations occur in most lower grade glioma and not only drive gliomagenesis but are also associated with longer patient survival and improved response to temozolomide. To investigate the possible causative relationship between these events, we introduced wild-type (WT) or mutant IDH1 into immortalized, untransformed human astrocytes, then monitored transformation status and temozolomide response. Temozolomide-sensitive parental cells exhibited DNA damage (γ-H2AX foci) and a prolonged G2 cell-cycle arrest beginning three days after temozolomide (100 μmol/L, 3 hours) exposure and persisting for more than four days. The same cells transformed by expression of mutant IDH1 exhibited a comparable degree of DNA damage and cell-cycle arrest, but both events resolved significantly faster in association with increased, rather than decreased, clonogenic survival. The increases in DNA damage processing, cell-cycle progression, and clonogenicity were unique to cells transformed by mutant IDH1, and were not noted in cells transformed by WT IDH1 or an oncogenic form (V12H) of Ras. Similarly, these effects were not noted following introduction of mutant IDH1 into Ras-transformed cells or established glioma cells. They were, however, associated with increased homologous recombination (HR) and could be reversed by the genetic or pharmacologic suppression of the HR DNA repair protein RAD51. These results show that mutant IDH1 drives a unique set of transformative events that indirectly enhance HR and facilitate repair of temozolomide-induced DNA damage and temozolomide resistance. The results also suggest that inhibitors of HR may be a viable means to enhance temozolomide response in IDH1-mutant glioma.

Sensitivity of glioblastomas to clinically available MEK inhibitors is defined by neurofibromin 1 deficiency

Loss of neurofibromin 1 (NF1) leads to hyperactivation of RAS, which in turn signals through the RAF/MEK/ERK and phosphoinositide 3-kinase (PI3K)/mTOR pathways to regulate cell growth and survival. Because NF1-deficient acute myeloid leukemias are sensitive to MEK inhibitors, we investigated here whether NF1-deficient glioblastoma multiforme (GBM) would respond to MEK inhibition. In 19 GBM cell lines, we found that treatment with the clinically available MEK inhibitors PD0325901 or AZD6244 decreased levels of phospho-ERK, the downstream effector of MEK, regardless of NF1 status. However, growth inhibition occurred only in a subset of NF1-deficient cells, in association with decreased levels of cyclin D1, increased levels of p27, and G1 arrest. As a single agent, PD0325901 suppressed the growth of NF1-deficient, MEK inhibitor-sensitive cells in vivo as well. Mechanistically, NF1-deficient, MEK inhibitor-sensitive cells were dependent upon the RAF/MEK/ERK pathway for growth and did not activate the PI3K pathway as a mechanism of acquired resistance. Importantly, NF1-deficient cells intrinsically resistant to MEK inhibition were sensitized by the addition of the dual PI3K/mTOR inhibitor PI-103. Taken together, our findings indicate that a subset of NF1-deficient GBMs may respond to MEK inhibitors currently being tested in clinical trials.

p27 deficiency is associated with migration defects in PDGF-expressing gliomas in vivo

p27(Kip1) is a cyclin dependent kinase inhibitor that functions as a tumor suppressor in a variety of different cancers. While p27 has a well established role in regulating the cell cycle, it has also been shown to regulate cellular migration by influencing the activation state of the small GTPase RhoA. We recently demonstrated that loss of p27 enhances tumor progression and leads to a dramatic decrease in survival in PDGF-induced oligodendrogliomas. Here we show that p27 deficient PDGF-expressing glial cells contained elevated levels of Rho-GTP and were less migratory than wild type cells. Migration defects in p27 deficient cells were rescued by either Rho kinase inhibition or expression of p27 or CK(-), a mutant of p27 that cannot bind cyclins/cdks. The RCAS/tv-a retroviral system was used to specifically induce PDGF-expressing gliomas in mice. Many of the p27 deficient mice died earlier than wild type mice and displayed hydrocephalus which was associated with periventricular tumors that failed to invade the normal brain parenchyma. Invasion failure was reversed by co-expression of PDGF with either the GAP domain of p190(RhoGAP), a negative regulator of Rho, or p27, or CK(-). These results suggest that p27 mediated regulation of the Rho pathway is cell cycle independent and demonstrate for the first time a migration defect in cancer cells that is associated with p27 deficiency in vivo in a mouse tumor model.