Translational studies of intravenous and intracerebroventricular routes of administration for CNS cellular biodistribution for BMN 250, an enzyme replacement therapy for the treatment of Sanfilippo type B.

BMN 250 is being developed as enzyme replacement therapy for Sanfilippo type B, a primarily neurological rare disease, in which patients have deficient lysosomal alpha-N-acetylglucosaminidase (NAGLU) enzyme activity. BMN 250 is taken up in target cells by the cation-independent mannose 6-phosphate receptor (CI-MPR, insulin-like growth factor 2 receptor), which then facilitates transit to the lysosome. BMN 250 is dosed directly into the central nervous system via the intracerebroventricular (ICV) route, and the objective of this work was to compare systemic intravenous (IV) and ICV delivery of BMN 250 to confirm the value of ICV dosing. We first assess the ability of enzyme to cross a potentially compromised blood-brain barrier in the Naglu-/- mouse model and then assess the potential for CI-MPR to be employed for receptor-mediated transport across the blood-brain barrier. In wild-type and Naglu-/- mice, CI-MPR expression in brain vasculature is high during the neonatal period but virtually absent by adolescence. In contrast, CI-MPR remains expressed through adolescence in non-affected non-human primate and human brain vasculature. Combined results from IV administration of BMN 250 in Naglu-/- mice and IV and ICV administration in healthy juvenile non-human primates suggest a limitation to therapeutic benefit from IV administration because enzyme distribution is restricted to brain vascular endothelial cells: enzyme does not reach target neuronal cells following IV administration, and pharmacological response following IV administration is likely restricted to clearance of substrate in endothelial cells. In contrast, ICV administration enables central nervous system enzyme replacement with biodistribution to target cells.

Differential Uptake of NAGLU-IGF2 and Unmodified NAGLU in Cellular Models of Sanfilippo Syndrome Type B.

Sanfilippo syndrome type B, or mucopolysaccharidosis IIIB (MPS IIIB), is a rare autosomal recessive lysosomal storage disease caused by a deficiency of α-N-acetylglucosaminidase (NAGLU). Deficiency in NAGLU disrupts the lysosomal turnover of heparan sulfate (HS), which results in the abnormal accumulation of partially degraded HS in cells and tissues. BMN 250 (NAGLU-insulin-like growth factor 2 [IGF2]) is a recombinant fusion protein developed as an investigational enzyme replacement therapy for MPS IIIB. The IGF2 peptide on BMN 250 promotes enhanced targeting of the enzyme to lysosomes through its interaction with the mannose 6-phosphate receptor. The focus of these studies was to further characterize the ability of NAGLU-IGF2 to clear accumulated HS compared to unmodified NAGLU in primary cellular models of MPS IIIB. Here, we establish distinct primary cell models of MPS IIIB with HS accumulation. These cellular models revealed distinct NAGLU uptake characteristics that depend on the duration of exposure. We found that with sustained exposure, NAGLU uptake and HS clearance occurred independent of known lysosomal targeting signals. In contrast, under conditions of limited exposure duration, NAGLU-IGF2 was taken up more rapidly than the unmodified NAGLU into MPS IIIB primary fibroblasts, astrocytes, and cortical neurons, where it efficiently degraded accumulated HS. These studies illustrate the importance of using physiologically relevant conditions in the evaluation of enzyme replacement therapies in cellular models.

BMN 250, a fusion of lysosomal alpha-N-acetylglucosaminidase with IGF2, exhibits different patterns of cellular uptake into critical cell types of Sanfilippo syndrome B disease pathogenesis.

Sanfilippo syndrome type B (Sanfilippo B; Mucopolysaccharidosis type IIIB) occurs due to genetic deficiency of lysosomal alpha-N-acetylglucosaminidase (NAGLU) and subsequent lysosomal accumulation of heparan sulfate (HS), which coincides with devastating neurodegenerative disease. Because NAGLU expressed in Chinese hamster ovary cells is not mannose-6-phosphorylated, we developed an insulin-like growth factor 2 (IGF2)-tagged NAGLU molecule (BMN 250; tralesinidase alfa) that binds avidly to the IGF2 / cation-independent mannose 6-phosphate receptor (CI-MPR) for glycosylation independent lysosomal targeting. BMN 250 is currently being developed as an investigational enzyme replacement therapy for Sanfilippo B. Here we distinguish two cellular uptake mechanisms by which BMN 250 is targeted to lysosomes. In normal rodent-derived neurons and astrocytes, the majority of BMN250 uptake over 24 hours reaches saturation, which can be competitively inhibited with IGF2, suggestive of CI-MPR-mediated uptake. Kuptake, defined as the concentration of enzyme at half-maximal uptake, is 5 nM and 3 nM in neurons and astrocytes, with a maximal uptake capacity (Vmax) corresponding to 764 nmol/hr/mg and 5380 nmol/hr/mg, respectively. Similar to neurons and astrocytes, BMN 250 uptake in Sanfilippo B patient fibroblasts is predominantly CI-MPR-mediated, resulting in augmentation of NAGLU activity with doses of enzyme that fall well below the Kuptake (5 nM), which are sufficient to prevent HS accumulation. In contrast, uptake of the untagged recombinant human NAGLU (rhNAGLU) enzyme in neurons, astrocytes and fibroblasts is negligible at the same doses tested. In microglia, receptor-independent uptake, defined as enzyme uptake resistant to competition with excess IGF2, results in appreciable lysosomal delivery of BMN 250 and rhNAGLU (Vmax = 12,336 nmol/hr/mg and 5469 nmol/hr/mg, respectively). These results suggest that while receptor-independent mechanisms exist for lysosomal targeting of rhNAGLU in microglia, BMN 250, by its IGF2 tag moiety, confers increased CI-MPR-mediated lysosomal targeting to neurons and astrocytes, two additional critical cell types of Sanfilippo B disease pathogenesis.

Utilizing ExAC to assess the hidden contribution of variants of unknown significance to Sanfilippo Type B incidence.

Given the large and expanding quantity of publicly available sequencing data, it should be possible to extract incidence information for monogenic diseases from allele frequencies, provided one knows which mutations are causal. We tested this idea on a rare, monogenic, lysosomal storage disorder, Sanfilippo Type B (Mucopolysaccharidosis type IIIB). Sanfilippo Type B is caused by mutations in the gene encoding α-N-acetylglucosaminidase (NAGLU). There were 189 NAGLU missense variants found in the ExAC dataset that comprises roughly 60,000 individual exomes. Only 24 of the 189 missense variants were known to be pathogenic; the remaining 165 variants were of unknown significance (VUS), and their potential contribution to disease is unknown. To address this problem, we measured enzymatic activities of 164 NAGLU missense VUS in the ExAC dataset and developed a statistical framework for estimating disease incidence with associated confidence intervals. We found that 25% of VUS decreased the activity of NAGLU to levels consistent with Sanfilippo Type B pathogenic alleles. We found that a substantial fraction of Sanfilippo Type B incidence (67%) could be accounted for by novel mutations not previously identified in patients, illustrating the utility of combining functional activity data for VUS with population-wide allele frequency data in estimating disease incidence.

Clearance of Heparan Sulfate and Attenuation of CNS Pathology by Intracerebroventricular BMN 250 in Sanfilippo Type B Mice.

Sanfilippo syndrome type B (mucopolysaccharidosis IIIB), caused by inherited deficiency of α-N-acetylglucosaminidase (NAGLU), required for lysosomal degradation of heparan sulfate (HS), is a pediatric neurodegenerative disorder with no approved treatment. Intracerebroventricular (ICV) delivery of a modified recombinant NAGLU, consisting of human NAGLU fused with insulin-like growth factor 2 (IGF2) for enhanced lysosomal targeting, was previously shown to result in marked enzyme uptake and clearance of HS storage in the Naglu-/- mouse brain. To further evaluate regional, cell type-specific, and dose-dependent biodistribution of NAGLU-IGF2 (BMN 250) and its effects on biochemical and histological pathology, Naglu-/- mice were treated with 1-100 µg ICV doses (four times over 2 weeks). 1 day after the last dose, BMN 250 (100 µg doses) resulted in above-normal NAGLU activity levels, broad biodistribution, and uptake in all cell types, with NAGLU predominantly localized to neurons in the Naglu-/- mouse brain. This led to complete clearance of disease-specific HS and reduction of secondary lysosomal defects and neuropathology across various brain regions lasting for at least 28 days after the last dose. The substantial brain uptake of NAGLU attainable by this highest ICV dosage was required for nearly complete attenuation of disease-driven storage accumulations and neuropathology throughout the Naglu-/- mouse brain.

Discovery and Characterization of (8S,9R)-5-Fluoro-8-(4-fluorophenyl)-9-(1-methyl-1H-1,2,4-triazol-5-yl)-2,7,8,9-tetrahydro-3H-pyrido[4,3,2-de]phthalazin-3-one (BMN 673, Talazoparib), a Novel, Highly Potent, and Orally Efficacious Poly(ADP-ribose) Polymerase-1/2 Inhibitor, as an Anticancer Agent.

We discovered and developed a novel series of tetrahydropyridophthlazinones as poly(ADP-ribose) polymerase (PARP) 1 and 2 inhibitors. Lead optimization led to the identification of (8S,9R)-47 (talazoparib; BMN 673; (8S,9R)-5-fluoro-8-(4-fluorophenyl)-9-(1-methyl-1H-1,2,4-triazol-5-yl)-2,7,8,9-tetrahydro-3H-pyrido[4,3,2-de]phthalazin-3-one). The novel stereospecific dual chiral-center-embedded structure of this compound has enabled extensive and unique binding interactions with PARP1/2 proteins. (8S,9R)-47 demonstrates excellent potency, inhibiting PARP1 and PARP2 enzyme activity with Ki = 1.2 and 0.87 nM, respectively. It inhibits PARP-mediated PARylation in a whole-cell assay with an EC50 of 2.51 nM and prevents proliferation of cancer cells carrying mutant BRCA1/2, with EC50 = 0.3 nM (MX-1) and 5 nM (Capan-1), respectively. (8S,9R)-47 is orally available, displaying favorable pharmacokinetic (PK) properties and remarkable antitumor efficacy in the BRCA1 mutant MX-1 breast cancer xenograft model following oral administration as a single-agent or in combination with chemotherapy agents such as temozolomide and cisplatin. (8S,9R)-47 has completed phase 1 clinical trial and is currently being studied in phase 2 and 3 clinical trials for the treatment of locally advanced and/or metastatic breast cancer with germline BRCA1/2 deleterious mutations.

Trapping Poly(ADP-Ribose) Polymerase.

Recent findings indicate that a major mechanism by which poly(ADP-ribose) polymerase (PARP) inhibitors kill cancer cells is by trapping PARP1 and PARP2 to the sites of DNA damage. The PARP enzyme-inhibitor complex "locks" onto damaged DNA and prevents DNA repair, replication, and transcription, leading to cell death. Several clinical-stage PARP inhibitors, including veliparib, rucaparib, olaparib, niraparib, and talazoparib, have been evaluated for their PARP-trapping activity. Although they display similar capacity to inhibit PARP catalytic activity, their relative abilities to trap PARP differ by several orders of magnitude, with the ability to trap PARP closely correlating with each drug's ability to kill cancer cells. In this article, we review the available data on molecular interactions between these clinical-stage PARP inhibitors and PARP proteins, and discuss how their biologic differences might be explained by the trapping mechanism. We also discuss how to use the PARP-trapping mechanism to guide the development of PARP inhibitors as a new class of cancer therapy, both for single-agent and combination treatments.

Neutral endopeptidase-resistant C-type natriuretic peptide variant represents a new therapeutic approach for treatment of fibroblast growth factor receptor 3-related dwarfism.

Achondroplasia (ACH), the most common form of human dwarfism, is caused by an activating autosomal dominant mutation in the fibroblast growth factor receptor-3 gene. Genetic overexpression of C-type natriuretic peptide (CNP), a positive regulator of endochondral bone growth, prevents dwarfism in mouse models of ACH. However, administration of exogenous CNP is compromised by its rapid clearance in vivo through receptor-mediated and proteolytic pathways. Using in vitro approaches, we developed modified variants of human CNP, resistant to proteolytic degradation by neutral endopeptidase, that retain the ability to stimulate signaling downstream of the CNP receptor, natriuretic peptide receptor B. The variants tested in vivo demonstrated significantly longer serum half-lives than native CNP. Subcutaneous administration of one of these CNP variants (BMN 111) resulted in correction of the dwarfism phenotype in a mouse model of ACH and overgrowth of the axial and appendicular skeletons in wild-type mice without observable changes in trabecular and cortical bone architecture. Moreover, significant growth plate widening that translated into accelerated bone growth, at hemodynamically tolerable doses, was observed in juvenile cynomolgus monkeys that had received daily subcutaneous administrations of BMN 111. BMN 111 was well tolerated and represents a promising new approach for treatment of patients with ACH.

Delivery of an enzyme-IGFII fusion protein to the mouse brain is therapeutic for mucopolysaccharidosis type IIIB.

Mucopolysaccharidosis type IIIB (MPS IIIB, Sanfilippo syndrome type B) is a lysosomal storage disease characterized by profound intellectual disability, dementia, and a lifespan of about two decades. The cause is mutation in the gene encoding α-N-acetylglucosaminidase (NAGLU), deficiency of NAGLU, and accumulation of heparan sulfate. Impediments to enzyme replacement therapy are the absence of mannose 6-phosphate on recombinant human NAGLU and the blood-brain barrier. To overcome the first impediment, a fusion protein of recombinant NAGLU and a fragment of insulin-like growth factor II (IGFII) was prepared for endocytosis by the mannose 6-phosphate/IGFII receptor. To bypass the blood-brain barrier, the fusion protein ("enzyme") in artificial cerebrospinal fluid ("vehicle") was administered intracerebroventricularly to the brain of adult MPS IIIB mice, four times over 2 wk. The brains were analyzed 1-28 d later and compared with brains of MPS IIIB mice that received vehicle alone or control (heterozygous) mice that received vehicle. There was marked uptake of the administered enzyme in many parts of the brain, where it persisted with a half-life of approximately 10 d. Heparan sulfate, and especially disease-specific heparan sulfate, was reduced to control level. A number of secondary accumulations in neurons [ß-hexosaminidase, LAMP1(lysosome-associated membrane protein 1), SCMAS (subunit c of mitochondrial ATP synthase), glypican 5, ß-amyloid, P-tau] were reduced almost to control level. CD68, a microglial protein, was reduced halfway. A large amount of enzyme also appeared in liver cells, where it reduced heparan sulfate and ß-hexosaminidase accumulation to control levels. These results suggest the feasibility of enzyme replacement therapy for MPS IIIB.

Structural basis for the inhibition of poly(ADP-ribose) polymerases 1 and 2 by BMN 673, a potent inhibitor derived from dihydropyridophthalazinone.

Poly(ADP-ribose) polymerases 1 and 2 (PARP1 and PARP2), which are involved in DNA damage response, are targets of anticancer therapeutics. BMN 673 is a novel PARP1/2 inhibitor with substantially increased PARP-mediated tumor cytotoxicity and is now in later-stage clinical development for BRCA-deficient breast cancers. In co-crystal structures, BMN 673 is anchored to the nicotinamide-binding pocket via an extensive network of hydrogen-bonding and π-stacking interactions, including those mediated by active-site water molecules. The novel di-branched scaffold of BMN 673 extends the binding interactions towards the outer edges of the pocket, which exhibit the least sequence homology among PARP enzymes. The crystallographic structural analyses reported here therefore not only provide critical insights into the molecular basis for the exceptionally high potency of the clinical development candidate BMN 673, but also new opportunities for increasing inhibitor selectivity.