Incidence and Clinical Presentation of Severe Neurotoxicity from Nelarabine in Patients with T-Cell Acute Lymphoblastic Leukemia.

BACKGROUND: Nelarabine is a purine analog with demonstrated efficacy in the treatment of T-cell Lymphoblastic Leukemia and Lymphoma (T-ALL/LBL). Despite its efficacy and excellent blood-brain barrier penetration, it has a significant side effect profile which is namely concerning for neurotoxicity. Reported neurotoxicity has varied from mild peripheral neuropathy to debilitating grade 4 neurologic complications including Guillain-Barre like syndrome and myelopathy. PATIENTS AND METHODS: We conducted a single centered, retrospective case series to study patients who developed severe neurotoxicity after receiving nelarabine as part of T-ALL treatment. One hundred thirty-five patients were identified. Thirteen patients were reviewed for severe neurotoxicity (defined as ≥grade 3), and of those five patients were deemed to have neurotoxicity secondary to nelarabine exposure. RESULTS: Five patients (4%) developed severe neurotoxicity as manifested by Guillain-Barre like syndrome or myelopathy within a timeframe of eight to fifty-eight days from last nelarabine dose. Upon diagnosis, patients received formal neurologic evaluation by our neuro-oncology specialists including imaging, cerebrospinal fluid testing, and electromyography. Patients were treated with IVIG, and steroids upon diagnosis, but the majority of neuro-deficits were irreversible. CONCLUSION: Our study shows that nelarabine is generally well-tolerated, and the incidence of severe neurotoxicity is rare. Given the potential risk of severe neurotoxicity, we propose capped dose of nelarabine 1000 mg/day, neurological assessment before subsequent dosing, and avoidance of simultaneous IT therapy during nelarabine administration.

Daratumumab and Nelarabine Treatment as Salvage Therapy for T-Lymphoblastic Lymphoma: A Case Report.

T-cell lymphoblastic lymphoma is an uncommon lymphoid neoplasm in adults, although more frequent in children and teenagers, that often affects the mediastinum and bone marrow, requiring intensive chemotherapy protocols. Its prognosis is poor if a cure is not achieved with first-line treatments. We present a case report of a 19-year-old man diagnosed with this type of lymphoma due to significant respiratory distress and a mediastinal mass. He received treatment according to the hyper-CVAD regimen, with a complete metabolic response. However, seven months later a new mediastinal growth was observed, leading to salvage treatment with a combination of nelarabine and daratumumab. We observed not only refractoriness, but also leukemization, which prompted consideration of hematopoietic stem cell transplantation. Based on this case, we conducted a review of pharmacological treatment options for refractory or relapsed lymphoblastic lymphoma, as well as the role of radiotherapy in managing mediastinal disease. This case report highlights the limited evidence available regarding later-line treatments, with unusual reports regarding employing our combination of daratumumab and nelarabine, and emphasizes the importance of achieving cures in the first line of treatment.

Nelarabine-containing salvage therapy and conditioning regimen in transplants for pediatric T-cell acute lymphoblastic leukemia and lymphoma.

Therapy for relapsed or refractory (r/r) T-cell acute lymphoblastic leukemia (T-ALL) and T-cell lymphoblastic lymphoma (T-LBL) in children is challenging, and new treatment methods are needed. We retrospectively analyzed eight patients with r/r T-ALL (five patients) and T-LBL (three patients) who were treated with nelarabine (NEL) plus etoposide, cyclophosphamide, and intrathecal therapy, administered 3 days apart. Five patients achieved a complete response, and the other three achieved a partial response (PR). All patients underwent hematopoietic stem cell transplantation (HSCT) after two cycles of treatment, except for one patient who received one cycle. Three patients who had previously received HSCT were treated with reduced-intensity conditioning regimens, including fludarabine, melphalan, and NEL; one survived for over 5 years after the second HSCT. Grade 2 neuropathy occurred in one patient, but other severe toxicities commonly associated with NEL were not observed during NEL administration in combination with chemotherapy. The 2-year overall survival and event-free survival rates were 60.0% and 36.5%, respectively. The addition of NEL to reinduction chemotherapy was useful in achieving remission and did not lead to excessive toxicity. In addition, a conditioning regimen, including NEL, appeared to be effective in patients who had previously undergone HSCT.

[Severe consciousness disturbance after cord blood transplantation for relapsed T lymphoblastic lymphoma].

T-lymphoblastic leukemia/lymphoma (T-ALL/LBL) has a poor prognosis. Nelarabine has recently shown relatively good results in patients with relapsed or refractory T-ALL/LBL, but requires careful monitoring for neurological complications. A 50-year-old man with early recurrence of T-LBL after allogenic peripheral blood stem cell transplantation received nelarabine monotherapy and achieved complete remission after 1 cycle. He then received umbilical cord blood transplantation, and experienced sustained disturbance of consciousness. He later died of multiple organ failure, and autopsy suggested that nelarabine-induced leukoencephalopathy had caused the disturbance of consciousness. This case suggests that physicians should carefully monitor patients for neurological complications and consider imaging follow-up and consultation with a neurologist.

Management of Nelarabine induced neurotoxicity in a child with T-cell acute lymphoblastic lymphoma.

INTRODUCTION: Nelarabine is now increasingly being used for the treatment of relapsed T-cell acute lymphoblastic leukemia/lymphoma, and about 18% of patients experience ≥ grade 3 toxicity. Despite the increasing use of this drug, there are no guidelines for managing its neurotoxicity. We would like to share our experience with one such case. CASE REPORT: A sixteen-year-old girl with T-lymphoblastic lymphoma received Nelarabine as part of her relapse treatment. Three weeks post-treatment, patient presented with worsening encephalopathy, bulbar palsy, and seizures. MANAGEMENT AND OUTCOME: After a detailed evaluation, Nelarabine neurotoxicity was strongly considered and was managed with a combination of steroids, intravenous immunoglobulin, and aminophylline, with almost complete recovery starting at 72 hours of treatment initiation. DISCUSSION: Despite the increasing use of this drug, guidelines for the management of the neurological adverse effects of Nelarabine are lacking. The above-mentioned combination of drugs worked for our patient, but larger numbers are needed to validate this as an approved treatment regimen.

The role of nelarabine in the treatment of T-cell acute lymphoblastic leukemia/lymphoma: challenges, opportunities, and future directions.

INTRODUCTION: Nelarabine is a guanine nucleoside analog and functions to terminate DNA synthesis in dividing cells. Pre-clinical and clinical studies have shown that it preferentially accumulates in T-cells where it exerts its cytotoxic effects. After generations of treatment protocol advances, it has been incorporated into numerous treatment regimens against T-lineage acute lymphoblastic leukemia/lymphoma (T-ALL/LLy). On 8 March 2023, the FDA approved the use of nelarabine for its use in T-ALL due to clear evidence of clinical benefits. This announcement concludes a nearly 6-decade period of evaluation for nelarabine and its role in the management of high-grade, aggressive T-cell malignancies. AREAS COVERED: We review the medicinal biology of nelarabine, its evaluation through decades of clinical studies, its dose-limited adverse effects, and its areas of highest impact in the treatment of T-ALL/LLy. EXPERT OPINION: We provide a context of when nelarabine might be considered in treatments against T-ALL/LLy, and also alternative strategies when it has or has not been used in therapies prior to relapse. We anticipate that an increasing number of treatment regimens will include nelarabine as a part of front-line therapy.

Successful stem cell transplantation after nelarabine, pegylated asparaginase, vincristine, doxorubicin, and prednisone in refractory early T-cell precursor acute lymphoblastic leukemia: A case report

Early T-cell precursor Acute Lymphoblastic Leukemia (ALL) has a dismal prognosis. Nelarabine is a purine nucleoside analog that increases the apoptosis rate in T-cell lymphoblasts. We present a 30-year-old patient diagnosed with T-cell ALL. He was a high-risk patient because of an early precursor phenotype and a complex karyotype that had been refractory to three previous lines of treatment. He started a course of nelarabine (1500 mg/m for three days), pegylated-asparaginase, doxorubicin, vincristine, and prednisone (Nelarabine Peg-Asp AdmVP). He reached complete remission and received an allogeneic sibling hematopoietic stem cell transplant with fludarabine, total body irradiation, and cyclophosphamide as the conditioning regimen. He developed a pulmonary mycosis, which resolved, and grade-2 neurotoxicity in his upper and lower limbs. He was discharged after 40 days and to date remains with 23 months of complete remission. The Nelarabine Peg-Asp AdmVP regimen seems to be effective and safe. Further research is needed to establish it as an induction treatment in refractory early T-cell precursor acute lymphoblastic leucemia.

Targeted treatment of T-cell acute lymphoblastic leukemia: latest updates from the 2022 ASH Annual Meeting.

T-cell acute lymphoblastic leukemia (T-ALL) occurs in approximately 25-30% of adult ALL. Currently, treatment approaches for adult patients with T-ALL remain quite limited, with intensive multiagent chemotherapy serving as the backbone; however, the cure rate remains unsatisfactory. Thus, the discovery of novel therapeutic strategies, especially targeted therapies, is crucial. Clinical research efforts are now focused on adding targeted therapy that has selective activity for T-ALL to the backbone chemotherapy regimen. To date, nelarabine remains the only targeted agent specifically approved for relapsed T-ALL, and the use of nelarabine in the first-line regimen is still being studied. Meanwhile, a number of novel targeted therapies with low toxicity, such as immunotherapies, are being actively investigated. Chimeric antigen receptor (CAR) T-cell therapy for the treatment of T-cell malignancies has not been as successful as in treating B-ALL due to fratricide. Numerous approaches are now being designed to address this challenge. Novel therapies targeting molecular aberrations in T-ALL are also actively investigated. T-ALL lymphoblasts overexpress BCL2 protein, which makes it an intriguing therapeutic target. This review summarizes the latest updates on targeted treatment of T-ALL from the 2022 ASH annual meeting.

Nelarabine-induced myelopathy in patients undergoing allogeneic hematopoietic cell transplantation: a report of three cases.

Nelarabine is an effective treatment for T-cell acute lymphoblastic leukemia/lymphoma. Myelopathy is a rare but serious adverse event associated with this drug. Three patients who received nelarabine at the National Cancer Center Hospital from December 2014 to March 2021 developed myelopathy 20 days before, 12 days after, and 29 days after allogeneic hematopoietic cell transplantation (allo-HCT), respectively. Magnetic resonance imaging showed that two of the patients had lesions in the dorsal column or medulla oblongata, and one had no abnormalities in the head or spine. Despite treatment with intravenous immunoglobulin and methylprednisolone, all patients became unable to walk. One patient died on day 101 after allo-HCT due to progressive neurotoxicity. The other two patients showed spontaneous improvement in neurological symptoms, but one died of mucormycosis on day 476. Autopsy revealed spongiosis in the posterior funiculus in both patients who died, and also in the medulla oblongata in one patient. In the surviving patient, positron emission tomography on day 84 showed abnormal accumulation, suggesting continued inflammation. These cases demonstrated pathophysiological features of nelarabine-induced myelopathy and indicate that allo-HCT may worsen the condition. It is necessary to elucidate the underlying mechanism and establish diagnostic methods and therapies.

Central nervous system neurotoxicity associated with nelarabine in T-cell acute lymphoblastic leukemia.

INTRODUCTION: Nelarabine, a prodrug of arabinosylguanine has lineage-specific toxicity for T lymphoblasts and is used to treat refractory or relapsed T-cell acute lymphoblastic leukemia and T-cell lymphoblastic lymphoma patients. The most commonly observed adverse effects associated with nelarabine are mainly hematological, i.e. neutropenia, anemia, and thrombocytopenia. Additionally, neurological, and gastrointestinal toxicities have been reported. Central nervous system neurotoxicity associated with nelarabine is very rare. CASE DESCRIPTION: A 37-year-old man patient diagnosed with T-cell acute lymphoblastic leukemia had experienced generalized tonic-clonic seizure which lasted for a few seconds and upper extremity weakness after three weeks of the nelarabine infusion. Computed tomography and magnetic resonance imaging have shown periventricular and nucleus caudatus abnormalities. Radiological findings suggested toxic leukoencephalopathy and acute infarct of right nucleus caudatus. MANAGEMENT AND OUTCOME: After high-dose steroids, intravenous immunoglobulin, and support treatment, his neurologic symptoms disappeared except for mild peroral numbness. However, radiological sequelae persisted despite clinical improvement. CONCLUSION: Physicians involved in the care of these patients who use nelarabine should be aware of the fact that cerebral toxicity of the nelarabine may occur especially in the presence of predisposing factors. It is crucial to monitor closely those patients receiving nelarabine and also those who have additional predisposing factors for neurotoxicity.

A phase 1 study to evaluate the safety, pharmacology, and feasibility of continuous infusion nelarabine in patients with relapsed and/or refractory lymphoid malignancies.

BACKGROUND: Nelarabine is a purine nucleoside analogue prodrug approved for the treatment of relapsed and refractory T-cell acute lymphoblastic leukemia (R/R T-ALL) and lymphoblastic lymphoma (T-LBL). Although effective in R/R T-ALL, significant neurotoxicity is dose-limiting and such neurotoxicity associated with nucleoside analogues can be related to dosing schedule. METHODS: The authors conducted a phase 1 study to evaluate the pharmacokinetics and toxicity of nelarabine administered as a continuous infusion (CI) for 5 days (120 hours), rather than the standard, short-infusion approach. RESULTS: Twenty-nine patients with R/R T-ALL/LBL or T-cell prolymphocytic leukemia (T-PLL) were treated, with escalating doses of nelarabine from 100 to 800 mg/m2 /day × 5 days. The median age of the patients was 39 years (range, 14-77 years). The overall response rate was 31%, including 27% complete remission (CR) or CR with incomplete platelet recovery (CRp). Peripheral neuropathy was observed in 34% of patients, including four ≥grade 3 events related to nelarabine. Notably, there was no nelarabine-related central neurotoxicity on study. The maximum tolerated dose was not reached. Pharmacokinetic data suggested no relationship between dose of nelarabine and accumulation of active intracellular ara-GTP metabolite. Higher intracellular ara-GTP concentrations were statistically associated with a favorable clinical response. CONCLUSION: Preliminary evaluation of continuous infusion schedule of nelarabine suggests that the safety profile is acceptable for this patient population, with clinical activity observed even at low doses and could broaden the use of nelarabine both as single agent and in combinations by potentially mitigating the risk of central nervous system toxicities.

The treatment of acute lymphoblastic leukemia in Jehovah's Witnesses and patients who cannot accept blood products.

Jehovah's Witnesses cannot accept blood products based upon religious beliefs, and when they present with acute leukemia, the ideal treatment strategy can be controversial. We present six cases of Jehovah's Witnesses with acute lymphoblastic leukemia and show that complete remission can be achieved without using anthracycline in 83% (5/6) of patients. We also report, for the first time in this population, that the use of agents with novel mechanisms of action, such as blinatumomab and nelarabine, is associated with minimal myelosuppression and can produce durable responses, with 2 of 6 patients still alive in CR3 at 4.9 and 6.6 years.

Nelarabine, etoposide, and cyclophosphamide in relapsed pediatric T-acute lymphoblastic leukemia and T-lymphoblastic lymphoma (study T2008-002 NECTAR).

Children with relapse of T-cell acute lymphoblastic leukemia (T-ALL) or lymphoblastic lymphoma (T-LBL) have a dismal prognosis, largely due to difficulty attaining second remission. We hypothesized that adding etoposide and cyclophosphamide to the nucleoside analog nelarabine could improve response rates over single-agent nelarabine for relapsed T-ALL and T-LBL. This phase I dose-escalation trial's primary objective was to evaluate the dose and safety of nelarabine given in combination with etoposide at 100 mg/m2 /day and cyclophosphamide at 330-400 mg/m2 /day, each for 5 consecutive days in children with either T-ALL (13 patients) or T-LBL (10 patients). Twenty-three patients were treated at three dose levels; 21 were evaluable for dose-limiting toxicities (DLT) and response. The recommended phase II doses (RP2D) for this regimen, when given daily ×5 every 3 weeks, were nelarabine 650 mg/m2 /day, etoposide 100 mg/m2 /day, and cyclophosphamide 400 mg/m2 /day. DLTs included peripheral motor and sensory neuropathies. An expansion cohort to evaluate responses at the RP2D was terminated early due to slow accrual. The overall best response rate was 38% (8/21), with 33% (4/12) responses in the T-ALL cohort and 44% (4/9) responses in the T-LBL cohort. These response rates are comparable to those seen with single-agent nelarabine in this setting. These data suggest that the addition of cyclophosphamide and etoposide to nelarabine does not increase the incidence of neurologic toxicities or the response rate beyond that obtained with single-agent nelarabine in children with first relapse of T-ALL and T-LBL.

[Treatment strategy for T-cell acute lymphoblastic leukemia in children].

T-cell acute lymphoblastic leukemia (T-ALL) accounted for approximately 10-15% of pediatric ALL and has often been treated within the same framework as B-cell precursor ALL (BCP-ALL). T-ALL has a poorer prognosis than BCP-ALL. However, improvements have been achieved through treatment intensification strategies using dexamethasone, L-asparaginase, and nelarabine, thereby reducing cranial irradiation. Furthermore, T-ALL-specific treatment protocols have been introduced based on these advancements. The JPLSG ALL-T11/JALSG T-ALL-211-U trial in Japan has been conducted from 2011 to 2017 for newly diagnosed patients with T-ALL under the age of 25 years. The trial included minimal residual disease-based treatment stratification and treatment intensification as described above and has shown excellent outcomes. Recently, new therapeutic agents have been actively developed for T-ALL. Thus, targeted therapy development based on new findings is expected in the future.

Nelarabine-induced rhabdomyolysis in a patient with T-cell acute lymphoblastic leukemia: a case report.

BACKGROUND: Nelarabine is an antineoplastic purine analog used for the treatment of refractory or relapsed T-cell acute lymphoblastic leukemia (T-ALL). The most prominent side effect of nelarabine are neurotoxicity and hematologic disorder, which are considered dose-limiting factors. Although clinical studies have reported myopathy due to nelarabine, actual detailed outcomes were not well-known initial approval. The incidence of nelarabine induced rhabdomyolysis has been reported at 2% in study in children. Cases of rhabdomyolysis have been reported in adults from medical facilities in the United Sates with renal dysfunction or severe muscle symptoms after administration of multiple courses of nelarabine. In this report, we discuss a case of rhabdomyolysis diagnosed after a single course of nelarabine. In this case, creatine kinase (CK) level was elevated in grade 4, without renal dysfunction and severe muscle symptoms. CASE PRESENTATION: A 46-year-old man from Japan was diagnosed with T-ALL and received a hematopoietic stem cell transplantation in first remission. However, the disease relapsed 6 months after transplantation. Nelarabine was selected as the next-line chemotherapeutic agent. The patient received 1500 mg/m2 of nelarabine on day 1 followed by a dose on days 3 and 5. CK levels, which were baseline before treatment, increased to grade 4 (18,620 IU/L) on the 8th day of treatment. He was diagnosed as rhabdomyolysis due to nelarabine with little possibility of other factors. He complained only of mild pain in his upper extremities and no other symptoms were noticed. The patient was managed with hydration. The pain lasted approximately 7 days, but there were no sequelae secondary to the rhabdomyolysis. Because of the elevation of CK in grade 4, we avoided re-administration. CONCLUSION: In the patient administrated nelarabine, CK level was elevated in grade 4, without other symptoms of rhabdomyolysis. The results suggest that CK may be elevated at the onset of rhabdomyolysis caused by nelarabine, even in the absence of other symptoms. Therefore, it was suggested that monitoring CK during nelarabine administration is important for detecting rhabdomyolysis before it becomes severe. We consider that CK should be monitored even in absence of symptoms.

Efficacy and safety of nelarabine in patients with relapsed or refractory T-cell acute lymphoblastic leukemia: a systematic review and meta-analysis.

Nelarabine is approved for the treatment of relapsed/refractory (R/R) T-cell acute lymphoblastic leukemia (T-ALL) patients who relapse following at least two different chemotherapy regimens. Previous studies have evaluated the efficacy and safety of nelarabine with chemotherapy in the treatment of R/R T-ALL. However, the results are inconsistent. This review aimed to summarize findings on efficacy and safety data in R/R T-ALL patients administered with the drug nelarabine. The present review conducted a comprehensive search of MEDLINE (via PubMed), WHO Clinical Trial Registry, Clinical Trials.gov, and Cochrane Central Register of Controlled Trials until 15 January 2022. Thirteen studies fulfilled the eligibility criteria with a total of 2508 patients. The efficacy of nelarabine was studied in terms of complete remission (CR) and partial remission (PR). Included studies reported overall random-effects pooled prevalence of CR and PR were 37.2 (95% CI: 22.8, 51.5) and 10.2 (95% CI: 4.9, 15.5), respectively. Most common adverse events associated with nelarabine were neutropenia, thrombocytopenia, fatigue, infections, and reversible peripheral neuropathy. Nelarabine is being used as salvage therapy as a bridge to hematopoietic stem cell transplantation and the findings of this meta-analysis indicate that it is an effective and safe treatment to be used in addition to the first-line treatment for R/R T-ALL.

Pharmacotherapeutic management of T-cell acute lymphoblastic leukemia in adults: an update of the literature.

INTRODUCTION: T-cell acute lymphoblastic leukemia (T-ALL) is a rare but potentially life-threatening heterogeneous hematologic malignancy that requires prompt diagnosis and treatment by hematologists. So far, therapeutic advances have been achieved in the management of this disease mainly by adopting pediatric-like regimens, and cure rates are significantly worse than in childhood. In T-ALL, less than 70% of adults achieve long-term survival. The prognosis after relapse is still very poor. Hence, there is urgent need to improve therapy of T-ALL by testing new compounds and combinations for the treatment of this disease. AREAS COVERED: This review provides a comprehensive update on the most recent treatment approaches in adults with de novo and relapsed/refractory adult T-ALL. EXPERT OPINION: Intensifying chemotherapy may reduce the incidence of recurrent disease in adult patients, but it has not come without a cost. Novel agents with selective T-ALL activity (e.g. nelarabine) may improve survival in some patient subsets. Due to modern genomic and transcriptomic techniques, various novel potential targets might change the treatment landscape in the next few years and will, hopefully alongside with cellular therapies, augment the therapeutic armamentarium in the near future.

Changes in intracellular activation-related gene expression and induction of Akt contribute to acquired resistance toward nelarabine in CCRF-CEM cell line.

Drug resistance is a major problem in treatment with nelarabine, and its resolution requires elucidation of the underlying mechanisms. We established two nelarabine-resistant subclones of the human T-cell lymphoblastic leukemia cell line CCRF-CEM. The resistant subclones showed changes in the expression of several genes related to nelarabine intracellular activation and inhibition of apoptosis. Activation of the Akt protein upon nelarabine treatment was observed in both subclones. The combination treatment with nelarabine and PI3K/Akt inhibitors was shown to inhibit cell growth. Cross-resistance was observed with ara-C and not with vincristine, daunorubicin, or etoposide treatment. Thus, changes in the expression of cellular activation-related genes, inhibition of apoptosis, and induction of Akt may be involved in the development of nelarabine resistance in the CCRF-CEM cell model. The use of different classes of chemotherapeutic agents and combination therapy with PI3K/Akt pathway inhibitors may be used to overcome resistance to nelarabine.

Drug resistance to nelarabine in leukemia cell lines might be caused by reduced expression of deoxycytidine kinase through epigenetic mechanisms.

PURPOSE: Drug resistance is a serious problem in leukemia therapy. A novel purine nucleoside analogue, nelarabine, is available for the treatment of children with T cell acute lymphoblastic leukemia. We investigated the mechanisms of drug resistance to nelarabine. METHODS: Nelarabine-resistant cells were selected by stepwise and continuous exposure to nelarabine using the limiting dilution method in human B and T cell lymphoblastic leukemia cell lines. Expression analysis was performed using real-time polymerase chain reaction, and epigenetic analysis was performed using methylation-specific polymerase chain reaction and chromatin immunoprecipitation. RESULTS: The RNA expression level for deoxycytidine kinase (dCK) was decreased in nelarabine-resistant leukemia cells. There were no differences between the parental and nelarabine-resistant leukemia cells in the methylation status of the promoter region of the dCK gene. In the chromatin immune precipitation assay, decreased acetylation of histones H3 and H4 bound to the dCK promoter was seen in the nelarabine-resistant cells when compared to the parental cells. Furthermore, treatment with a novel histone deacetylase inhibitor, vorinostat, promoted the cytotoxic effect of nelarabine along with increased expression of the dCK gene, and it increased acetylation of both histones H3 and H4 bound to the dCK promoter in nelarabine-resistant leukemia cells. The combination index showed that the effect of nelarabine and vorinostat was synergistic. CONCLUSION: This study reports that nelarabine with vorinostat can promote cytotoxicity in nelarabine-resistant leukemia cells through epigenetic mechanisms.