Tralesinidase Alfa Enzyme Replacement Therapy Prevents Disease Manifestations in a Canine Model of Mucopolysaccharidosis Type IIIB.

Mucopolysaccharidosis type IIIB (MPS IIIB; Sanfilippo syndrome B; OMIM #252920) is a lethal, pediatric, neuropathic, autosomal recessive, and lysosomal storage disease with no approved therapy. Patients are deficient in the activity of N-acetyl-alpha-glucosaminidase (NAGLU; EC 3.2.150), necessary for normal lysosomal degradation of the glycosaminoglycan heparan sulfate (HS). Tralesinidase alfa (TA), a fusion protein comprised of recombinant human NAGLU and a modified human insulin-like growth factor 2, is in development as an enzyme replacement therapy that is administered via intracerebroventricular (ICV) infusion, thus circumventing the blood brain barrier. Previous studies have confirmed ICV infusion results in widespread distribution of TA throughout the brains of mice and nonhuman primates. We assessed the long-term tolerability, pharmacology, and clinical efficacy of TA in a canine model of MPS IIIB over a 20-month study. Long-term administration of TA was well tolerated as compared with administration of vehicle. TA was widely distributed across brain regions, which was confirmed in a follow-up 8-week pharmacokinetic/pharmacodynamic study. MPS IIIB dogs treated for up to 20 months had near-normal levels of HS and nonreducing ends of HS in cerebrospinal fluid and central nervous system (CNS) tissues. TA-treated MPS IIIB dogs performed better on cognitive tests and had improved CNS pathology and decreased cerebellar volume loss relative to vehicle-treated MPS IIIB dogs. These findings demonstrate the ability of TA to prevent or limit the biochemical, pathologic, and cognitive manifestations of canine MPS IIIB disease, thus providing support of its potential long-term tolerability and efficacy in MPS IIIB subjects. SIGNIFICANCE STATEMENT: This work illustrates the efficacy and tolerability of tralesinidase alfa as a potential therapeutic for patients with mucopolysaccharidosis type IIIB (MPS IIIB) by documenting that administration to the central nervous system of MPS IIIB dogs prevents the accumulation of disease-associated glycosaminoglycans in lysosomes, hepatomegaly, cerebellar atrophy, and cognitive decline.

An anti-CD3/anti-CLL-1 bispecific antibody for the treatment of acute myeloid leukemia.

Acute myeloid leukemia (AML) is a major unmet medical need. Most patients have poor long-term survival, and treatment has not significantly changed in 40 years. Recently, bispecific antibodies that redirect the cytotoxic activity of effector T cells by binding to CD3, the signaling component of the T-cell receptor, and a tumor target have shown clinical activity. Notably, blinatumomab is approved to treat relapsed/refractory acute lymphoid leukemia. Here we describe the design, discovery, pharmacologic activity, pharmacokinetics, and safety of a CD3 T cell-dependent bispecific (TDB) full-length human IgG1 therapeutic antibody targeting CLL-1 that could potentially be used in humans to treat AML. CLL-1 is prevalent in AML and, unlike other targets such as CD33 and CD123, is not expressed on hematopoietic stem cells providing potential hematopoietic recovery. We selected a high-affinity monkey cross-reactive anti-CLL-1 arm and tested several anti-CD3 arms that varied in affinity, and determined that the high-affinity CD3 arms were up to 100-fold more potent in vitro. However, in mouse models, the efficacy differences were less pronounced, probably because of prolonged exposure to TDB found with lower-affinity CD3 TDBs. In monkeys, assessment of safety and target cell depletion by the high- and low-affinity TDBs revealed that only the low-affinity CD3/CLL1 TDB was well tolerated and able to deplete target cells. Our data suggest that an appropriately engineered CLL-1 TDB could be effective in the treatment of AML.

Nonclinical evaluation of CNS-administered TPP1 enzyme replacement in canine CLN2 neuronal ceroid lipofuscinosis.

The CLN2 form of neuronal ceroid lipofuscinosis, a type of Batten disease, is a lysosomal storage disorder caused by a deficiency of the enzyme tripeptidyl peptidase-1 (TPP1). Patients exhibit progressive neurodegeneration and loss of motor, cognitive, and visual functions, leading to death by the early teenage years. TPP1-null Dachshunds recapitulate human CLN2 disease. To characterize the safety and pharmacology of recombinant human (rh) TPP1 administration to the cerebrospinal fluid (CSF) as a potential enzyme replacement therapy (ERT) for CLN2 disease, TPP1-null and wild-type (WT) Dachshunds were given repeated intracerebroventricular (ICV) infusions and the pharmacokinetic (PK) profile, central nervous system (CNS) distribution, and safety were evaluated. TPP1-null animals and WT controls received 4 or 16mg of rhTPP1 or artificial cerebrospinal fluid (aCSF) vehicle every other week. Elevated CSF TPP1 concentrations were observed for 2-3 days after the first ICV infusion and were approximately 1000-fold higher than plasma levels at the same time points. Anti-rhTPP1 antibodies were detected in CSF and plasma after repeat rhTPP1 administration, with titers generally higher in TPP1-null than in WT animals. Widespread brain distribution of rhTPP1 was observed after chronic administration. Expected histological changes were present due to the CNS delivery catheters and were similar in rhTPP1 and vehicle-treated animals, regardless of genotype. Neuropathological evaluation demonstrated the clearance of lysosomal storage, preservation of neuronal morphology, and reduction in brain inflammation with treatment. This study demonstrates the favorable safety and pharmacology profile of rhTPP1 ERT administered directly to the CNS and supports clinical evaluation in patients with CLN2 disease.

Enzyme replacement therapy attenuates disease progression in a canine model of late-infantile neuronal ceroid lipofuscinosis (CLN2 disease).

Using a canine model of classical late-infantile neuronal ceroid lipofuscinosis (CLN2 disease), a study was conducted to evaluate the potential pharmacological activity of recombinant human tripeptidyl peptidase-1 (rhTPP1) enzyme replacement therapy administered directly to the cerebrospinal fluid (CSF). CLN2 disease is a hereditary neurodegenerative disorder resulting from mutations in CLN2, which encodes the soluble lysosomal enzyme tripeptidyl peptidase-1 (TPP1). Infants with mutations in both CLN2 alleles develop normally but in the late-infantile/early-childhood period undergo progressive neurological decline accompanied by pronounced brain atrophy. The disorder, a form of Batten disease, is uniformly fatal, with clinical signs starting between 2 and 4 years of age and death usually occurring by the early teenage years. Dachshunds homozygous for a null mutation in the canine ortholog of CLN2 (TPP1) exhibit a similar disorder that progresses to end stage at 10.5-11 months of age. Administration of rhTPP1 via infusion into the CSF every other week, starting at approximately 2.5 months of age, resulted in dose-dependent significant delays in disease progression, as measured by delayed onset of neurologic deficits, improved performance on a cognitive function test, reduced brain atrophy, and increased life span. Based on these findings, a clinical study evaluating the potential therapeutic value of rhTPP1 administration into the CSF of children with CLN2 disease has been initiated.

Recombinant human tripeptidyl peptidase-1 infusion to the monkey CNS: safety, pharmacokinetics, and distribution.

CLN2 disease is caused by deficiency in tripeptidyl peptidase-1 (TPP1), leading to neurodegeneration and death. The safety, pharmacokinetics (PK), and CNS distribution of recombinant human TPP1 (rhTPP1) were characterized following a single intracerebroventricular (ICV) or intrathecal-lumbar (IT-L) infusion to cynomolgus monkeys. Animals received 0, 5, 14, or 20mg rhTPP1, ICV, or 14 mg IT-L, in artificial cerebrospinal fluid (aCSF) vehicle. Plasma and CSF were collected for PK analysis. Necropsies occurred at 3, 7, and 14 days post-infusion. CNS tissues were sampled for rhTPP1 distribution. TPP1 infusion was well tolerated and without effect on clinical observations or ECG. A mild increase in CSF white blood cells (WBCs) was detected transiently after ICV infusion. Isolated histological changes related to catheter placement and infusion were observed in ICV treated animals, including vehicle controls. The CSF and plasma exposure profiles were equivalent between animals that received an ICV or IT-L infusion. TPP1 levels peaked at the end of infusion, at which point the enzyme was present in plasma at 0.3% to 0.5% of CSF levels. TPP1 was detected in brain tissues with half-lives of 3-14 days. CNS distribution between ICV and IT-L administration was similar, although ICV resulted in distribution to deep brain structures including the thalamus, midbrain, and striatum. Direct CNS infusion of rhTPP1 was well tolerated with no drug related safety findings. The favorable nonclinical profile of ICV rhTPP1 supports the treatment of CLN2 by direct administration to the CNS.

Enzyme replacement therapy delays pupillary light reflex deficits in a canine model of late infantile neuronal ceroid lipofuscinosis.

Late-infantile neuronal ceroid lipofuscinosis (CLN2 disease) is a hereditary neurological disorder characterized by progressive retinal degeneration and vision loss, cognitive and motor decline, seizures, and pronounced brain atrophy. This fatal pediatric disease is caused by mutations in the CLN2 gene which encodes the lysosomal enzyme tripeptidyl peptidase-1 (TPP1). Utilizing a TPP1-/- Dachshund model of CLN2 disease, studies were conducted to assess the effects of TPP1 enzyme replacement administered directly to the CNS on disease progression. Recombinant human TPP1 (rhTPP1) or artificial cerebrospinal fluid vehicle was administered to CLN2-affected dogs via infusion into the CSF. Untreated and vehicle treated affected dogs exhibited progressive declines in pupillary light reflexes (PLRs) and electroretinographic (ERG) responses to light stimuli. Studies were undertaken to determine whether CSF administration of rhTPP1 alters progression of the PLR and ERG deficits in the canine model. rhTPP1 administration did not inhibit the decline in ERG responses, as rhTPP1 treated, vehicle treated, and untreated dogs all exhibited similar progressive and profound declines in ERG amplitudes. However, in some of the dogs treated with rhTPP1 there were substantial delays in the appearance and progression of PLR deficits compared with untreated or vehicle treated affected dogs. These findings indicate that CSF administration of TPP1 can attenuate functional impairment of neural pathways involved in mediating the PLR but does not prevent loss of retinal responses detectable with ERG.

Intrathecal tripeptidyl-peptidase 1 reduces lysosomal storage in a canine model of late infantile neuronal ceroid lipofuscinosis.

Late infantile neuronal ceroid lipofuscinosis (LINCL) is caused by mutations in the gene encoding tripeptidyl-peptidase 1 (TPP1). LINCL patients accumulate lysosomal storage materials in the CNS accompanied by neurodegeneration, blindness, and functional decline. Dachshunds homozygous for a null mutation in the TPP1 gene recapitulate many symptoms of the human disease. The objectives of this study were to determine whether intrathecal (IT) TPP1 treatment attenuates storage accumulation and functional decline in TPP1-/- Dachshunds and to characterize the CNS distribution of TPP1 activity. TPP1 was administered to one TPP1-/- and one homozygous wild-type (WT) dog. An additional TPP1-/- and WT dog received vehicle. Four IT administrations of 32 mg TPP1 formulated in 2.3 mL of artificial cerebrospinal fluid (aCSF) or vehicle were administered monthly via the cerebellomedullary cistern from four to seven months of age. Functional decline was assessed by physical and neurological examinations, electrophysiology, and T-maze performance. Neural tissues were collected 48 h after the fourth administration and analyzed for TPP1 activity and autofluorescent storage material. TPP1 was distributed at greater than WT levels in many areas of the CNS of the TPP1-/- dog administered TPP1. The amount of autofluorescent storage was decreased in this dog relative to the vehicle-treated affected control. No improvement in overall function was observed in this dog compared to the vehicle-treated TPP1-/- littermate control. These results demonstrate for the first time in a large animal model of LINCL widespread delivery of biochemically active TPP1 to the brain after IT administration along with a decrease in lysosomal storage material. Further studies with this model will be necessary to optimize the dosing route and regimen to attenuate functional decline.

Dose selection for intracerebroventricular cerliponase alfa in children with CLN2 disease, translation from animal to human in a rare genetic disease.

Neuronal ceroid lipofuscinosis type 2 (CLN2 disease) is an ultra-rare pediatric neurodegenerative disorder characterized by deficiency of the lysosomal enzyme tripeptidyl peptidase-1 (TPP1). In the absence of adequate TPP1, lysosomal storage material accumulation occurs in the central nervous system (CNS) accompanied by neurodegeneration and neurological decline that culminates in childhood death. Cerliponase alfa is a recombinant human TPP1 enzyme replacement therapy administered via intracerebroventricular infusion and approved for the treatment of CLN2 disease. Here, we describe two allometric methods, calculated by scaling brain mass across species, that informed the human dose selection and exposure prediction of cerliponase alfa from preclinical studies in monkeys and a dog model of CLN2 disease: (1) scaling of dose using a human-equivalent dose factor; and (2) scaling of compartmental pharmacokinetic (PK) model parameters. Source PK data were obtained from cerebrospinal fluid (CSF) samples from dogs and monkeys, and the human exposure predictions were confirmed with CSF data from the first-in-human clinical study. Nonclinical and clinical data were analyzed using noncompartmental analysis and nonlinear mixed-effect modeling approaches. Both allometric methods produced CSF exposure predictions within twofold of the observed exposure parameters maximum plasma concentration (Cmax ) and area under the curve (AUC). Furthermore, cross-species qualification produced consistent and reasonable PK profile predictions, which supported the allometric scaling of model parameters. The challenges faced in orphan drug development place an increased importance on, and opportunity for, data translation from research and nonclinical development. Our approach to dose translation and human exposure prediction for cerliponase alfa may be applicable to other CNS administered therapies being developed.

Preclinical pharmacokinetics and pharmacodynamics of DCLL9718A: An antibody-drug conjugate for the treatment of acute myeloid leukemia.

Few treatment options are available for acute myeloid leukemia (AML) patients. DCLL9718A is an antibody-drug conjugate that targets C-type lectin-like molecule-1 (CLL-1). This receptor is prevalent on monocytes, neutrophils, and AML blast cells, and unlike CD33, is not expressed on hematopoietic stem cells, thus providing possible hematopoietic recovery. DCLL9718A comprises an anti-CLL-1 IgG1 antibody (MCLL0517A) linked to a pyrrolobenzodiazepine (PBD) dimer payload, via a cleavable disulfide-labile linker. Here, we characterize the in vitro and in vivo stability, the pharmacokinetics (PK) and pharmacodynamics (PD) of DCLL9718A and MCLL0517A in rodents and cynomolgus monkeys. Three key PK analytes were measured in these studies: total antibody, antibody-conjugated PBD dimer and unconjugated PBD dimer. In vitro, DCLL9718A, was stable with most (> 80%) of the PBD dimer payload remaining conjugated to the antibody over 96 hours. This was recapitulated in vivo with antibody-conjugated PBD dimer clearance estimates similar to DCLL9718A total antibody clearance. Both DCLL9718A and MCLL0517A showed linear PK in the non-binding rodent species, and non-linear PK in cynomolgus monkeys, a binding species. The PK data indicated minimal impact of conjugation on the disposition of DCLL9718A total antibody. Finally, in cynomolgus monkey, MCLL0517A showed target engagement at all doses tested (0.5 and 20 mg/kg) as measured by receptor occupancy, and DCLL9718A (at doses of 0.05, 0.1 and 0.2 mg/kg) showed strong PD activity as evidenced by notable reduction in monocytes and neutrophils.

Gene promoter hypermethylation in mouse lung tumors.

The mouse is a good model for evaluating the efficacy of chemopreventive agents for lung cancer. Gene silencing by promoter hypermethylation is a critical component for the development and progression of lung cancer and an emerging target for preventive intervention by demethylating agents. Genes methylated in mouse lung tumors could serve as biomarkers to evaluate the effectiveness of demethylating agents for preventing lung cancer and causing gene reexpression in vivo. The purpose of the current study was to evaluate a panel of genes inactivated by promoter hypermethylation in human lung cancer for silencing by this epigenetic mechanism in murine lung tumors induced by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), cigarette smoke, or arising spontaneously. Cadherin-13, estrogen receptor-alpha, progesterone receptor, and runt-related transcription factor-3 were frequently methylated in mouse lung tumor-derived cell lines, whereas cadherin-1 and suppressor of cytokine signaling-1 were not. Methylation within these four genes was associated with lack of expression that could be restored after treatment with 5-aza-2'-deoxycytidine and with methylation within the CpG island of each gene. Methylation-specific PCR revealed that methylation of these four genes occurred at prevalences of 24% to 69% in primary lung tumors arising spontaneously or induced by exposure to cigarette smoke or NNK. Estrogen receptor-alpha methylation was more frequent in spontaneously occurring lung cancer than cigarette smoke-induced or NNK-induced lung cancer, whereas runt-related transcription factor-3 showed the opposite relationship. Thus, genes can be targeted for inactivation by methylation, depending on exposure history. This study indicates that methylation events frequently observed in human lung cancer are recapitulated in the mouse model and identifies four potential biomarkers for assessing intervention approaches for reversing epigenetically mediated gene silencing.

Aberrant promoter hypermethylation of the death-associated protein kinase gene is early and frequent in murine lung tumors induced by cigarette smoke and tobacco carcinogens.

Loss of expression of the death-associated protein (DAP)-kinase gene by aberrant promoter methylation may play an important role in cancer development and progression. The purpose of this investigation was to determine the commonality for inactivation of the DAP-kinase gene in adenocarcinomas induced in mice by chronic exposure to mainstream cigarette smoke, the tobacco carcinogens 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and vinyl carbamate, and the occupational carcinogen methylene chloride. The timing for inactivation was also determined in alveolar hyperplasias that arise in lung cancer induced in the A/J mouse by NNK. The DAP-kinase gene was not expressed in three of five NNK-induced lung tumor-derived cell lines or in a spontaneously arising lung tumor-derived cell line. Treatment with 5-aza-2'-deoxycytidine restored expression; dense methylation throughout the DAP-kinase CpG island detected by bisulfite sequencing supported methylation as the inactivating event in these cell lines. Methylation-specific PCR detected inactivation of the DAP-kinase gene in 43% of tumors associated with cigarette smoke, a frequency similar to those reported in human non-small cell lung cancer. In addition, DAP-kinase methylation was detected in 52%, 60%, and 50% of tumors associated with NNK, vinyl carbamate, and methylene chloride, respectively. Methylation was observed at similar prevalence in both NNK-induced hyperplasias and adenocarcinomas (46% versus 52%), suggesting that inactivation of this gene is one pathway for tumor development in the mouse lung. Bisulfite sequencing of both premalignant and malignant lesions revealed dense methylation, substantiating that this gene is functionally inactivated at the earliest histological stages of adenocarcinoma development. This study is the first to use a murine model of cigarette smoke-induced lung cancer and demonstrate commonality for inactivation by promoter hypermethylation of a gene implicated in the development of this disease in humans.

Safety Evaluation of CNS Administered Biologics-Study Design, Data Interpretation, and Translation to the Clinic.

Many central nervous system (CNS) diseases are inadequately treated by systemically administered therapies due to the blood brain barrier (BBB), which prevents achieving adequate drug concentrations at sites of action. Due to the increasing prevalence of neurodegenerative diseases and the inability of most systemically administered therapies to cross the BBB, direct CNS delivery will likely play an increasing role in treatment. Administration of large molecules, cells, viral vectors, oligonucleotides, and other novel therapies directly to the CNS via the subarachnoid space, ventricular system, or parenchyma overcomes this obstacle. Clinical experience with direct CNS administration of small molecule therapies suggests that this approach may be efficacious for the treatment of neurodegenerative disorders using biological therapies. Risks of administration into the brain tissue or cerebrospinal fluid include local damage from implantation of the delivery system and/or administration of the therapeutic and reactions affecting the CNS. Preclinical safety studies on CNS administered compounds must differentiate between the effects of the test article, the delivery device, and/or the vehicle, and assess exacerbations of reactions due to combinations of effects. Animal models characterized for safety assessment of CNS administered therapeutics have enabled human trials, but interpretation can be challenging. This manuscript outlines the challenges of preclinical intrathecal/intracerebroventricular/intraparenchymal studies, evaluation of results, considerations for special endpoints, and translation of preclinical findings to enable first-in-human trials. Recommendations will be made based on the authors' collective experience with conducting these studies to enable clinical development of CNS-administered biologics.

Life-span inhalation exposure to mainstream cigarette smoke induces lung cancer in B6C3F1 mice through genetic and epigenetic pathways.

Although cigarette smoke has been epidemiologically associated with lung cancer in humans for many years, animal models of cigarette smoke-induced lung cancer have been lacking. This study demonstrated that life time whole body exposures of female B6C3F1 mice to mainstream cigarette smoke at 250 mg total particulate matter/m(3) for 6 h per day, 5 days a week induces marked increases in the incidence of focal alveolar hyperplasias, pulmonary adenomas, papillomas and adenocarcinomas. Cigarette smoke-exposed mice (n = 330) had a 10-fold increase in the incidence of hyperplastic lesions, and a 4.6-fold (adenomas and papillomas), 7.25-fold (adenocarcinomas) and 5-fold (metastatic pulmonary adenocarcinomas) increase in primary lung neoplasms compared with sham-exposed mice (n = 326). Activating point mutations in codon 12 of the K-ras gene were identified at a similar rate in tumors from sham-exposed mice (47%) and cigarette smoke-exposed mice (60%). The percentages of transversion and transition mutations were similar in both the groups. Hypermethylation of the death associated protein (DAP)-kinase and retinoic acid receptor (RAR)-beta gene promoters was detected in tumors from both sham- and cigarette smoke-exposed mice, with a tendency towards increased frequency of RAR-beta methylation in the tumors from the cigarette smoke-exposed mice. These results emphasize the importance of the activation of K-ras and silencing of DAP-kinase and RAR-beta in lung cancer development, and confirm the relevance of this mouse model for studying lung tumorigenesis.

Lymphoid tissue and emphysema in the lungs of transgenic mice inducibly expressing tumor necrosis factor-alpha.

To develop a model in which the pathogenic effects of the proinflammatory cytokine tumor necrosis factor-alpha (TNF) could be investigated, transgenic mice that express TNF in the lung under the control of a doxycycline-inducible promoter were generated. TNF transgene message was expressed at a low level in the absence of doxycycline treatment and was induced in the lung by administration of the drug. Analysis of lung lavage fluid indicated increases in neutrophils and lymphocytes in doxycycline-treated transgenic mice. Histologic analysis of lungs from adult transgenic mice treated with doxycycline revealed prominent development of lymphoid tissue and increases in airspace size. Genes upregulated in TNF transgenic mice, as identified by oligonucleotide microarray analysis, included a variety of transcripts expressed in lymphoid tissues. Immunohistochemical analysis demonstrated the presence of B lymphocytes and, to a lesser extent, T lymphocytes within lymphoid aggregates in TNF transgenic mice. CD8-positive T cells were absent from lymphocytic nodules, but in the lung parenchyma were more abundant in transgenic than in nontransgenic mice. These results indicate that induction of TNF in adult lung promotes the formation of lymphoid tissue and emphysema, and provides a model in which the pathogenic effects of TNF on the lung can be investigated.

Carcinogen exposure differentially modulates RAR-beta promoter hypermethylation, an early and frequent event in mouse lung carcinogenesis.

The retinoic acid receptor beta (RAR-beta) gene encodes one of the primary receptors for retinoic acid, an important signaling molecule in lung growth, differentiation and carcinogenesis. RAR-beta has been shown to be down-regulated by methylation in human lung cancer. We have used previously lung tumors induced in mice to evaluate the timing and effect of specific carcinogen exposures on targeting genes altered in human lung cancer. These studies were extended to characterize the role of methylation of the RAR-beta gene in murine lung cancers. After treatment with the demethylating agent 5-aza-2'-deoxycytidine (DAC), RAR-beta was re-expressed in silenced cell lines or expressed at a higher rate than without DAC, supporting methylation as the inactivating mechanism. Bisulfite sequencing detected dense methylation in the area of the CpG island that contained the 5' untranslated region and the first translated exon in non-expressing cell lines, compared with minimal and heterogeneous methylation in normal mouse lung. Methylation-specific PCR revealed that this gene is targeted differentially by carcinogen exposures with the detection of methylated alleles in virtually all primary tumors associated with cigarette smoke or 4-methylnitrosamino-1-(3-pyridyl)-butanone (NNK) in contrast to half of tumors induced by methylene chloride or vinyl carbamate. RAR-beta methylation was also detected in 54% of preneoplastic hyperplasias induced by treatment with NNK. Bisulfite sequencing of both premalignant and malignant lesions detected dense methylation in the same area observed in cell lines, substantiating that this gene is functionally inactivated at the earliest histologic stage of adenocarcinoma development. These studies demonstrate that aberrant methylation of RAR-beta is an early and common alteration in murine lung tumors induced by several environmentally relevant exposures.