CONSPECTUS Metal–organic frameworks (MOFs), a class of hybrid materials formed by the self-assembly of polydentate bridging ligands and metal-connecting points, have been studied for a variety of applications. Recently, these materials have been scaled down to nanometer sizes, and this Account details the development of nanoscale metal–organic frameworks (NMOFs) for biomedical applications. NMOFs possess several potential advantages over conventional nanomedicines such as their structural and chemical diversity, their high loading capacity, and their intrinsic biodegradability. Under relatively mild conditions, NMOFs can be obtained as either crystalline or amorphous materials. The particle composition, size, and morphology can be easily tuned to optimize the final particle properties. Researchers have employed two general strategies to deliver active agents using NMOFs: by incorporating active agents into the frameworks or by loading active agents into the pores and channels of the NMOFs. The modification of NMOF surfaces with either silica coatings or organic polymers improves NMOF stability, fine-tunes their properties, and imparts additional functionality. Preliminary biomedical applications of NMOFs have focused on their use as delivery vehicles for imaging contrast agents and molecular therapeutics. Because NMOFs can carry large amounts of paramagnetic metal ions, they have been extensively explored as magnetic resonance imaging (MRI) contrast agents. Both Gd3+- and Mn2+-containing NMOFs have shown excellent efficacy as T1-weighted contrast agents with large per metal- and per particle-based MR relaxivities. Fe3+-containing NMOFs have demonstrated excellent T2-weighted contrast enhancement. Upon intravenous injection of iron carboxylate NMOFs in Wistar rats, researchers observed negative signal enhancement in the liver and spleen, which dissipated over time, indicating the degradation and clearance of the NMOF. Through the incorporation of luminescent or high Z element building blocks, NMOFs have also served as viable contrast agents for optical imaging or X-ray computed tomography (CT) imaging. Incorporation of membrane impermeable dyes into NMOFs allowed for their uptake by cancer cells and for their controlled release as the framework decomposed. NMOFs have been used to deliver anticancer drugs and other chemotherapeutics. Cisplatin prodrugs were incorporated within NMOFs at exceptionally high levels, either through use of the prodrug as the building block or through attachment of the prodrug onto the framework after synthesis. These NMOFs were encapsulated within a silica shell and targeted to cancer cells. In vitro assays revealed that the targeted NMOFs possessed similar efficacy to cisplatin, while the nontargeted NMOFs were less active. Several different therapeutic molecules were loaded within porous iron-carboxylate NMOFs at unprecedented levels. The NMOF showed sustained drug release with no burst effect, and in vitro assays revealed that the nanoencapsulated drug possessed similar efficacy to...

Fe(III)-carboxylate nanoscale metal-organic frameworks (NMOFs) with the MIL-101 structure were synthesized using a solvothermal technique with microwave heating. The ~200 nm particles were characterized using a variety of methods, including SEM, PXRD, nitrogen adsorption measurements, TGA, and EDX. By replacing a percentage of the bridging ligand (terephthalic acid) with 2-amino terephthalic acid, amine groups were incorporated into the framework to provide sites for covalent attachment of biologically relevant cargoes while still maintaining the MIL-101 structure. In proof-of-concept experiments, an optical contrast agent (a BODIPY dye) and an ethoxysuccinato-cisplatin anticancer prodrug were successfully incorporated into the Fe(III)-carboxylate NMOFs via post-synthetic modifications of the as-synthesized particles. These cargoes are released upon the degradation of the NMOF frameworks, and the rate of cargo release was controlled by coating the NMOF particles with a silica shell. Potential utility of the new NMOF-based nano-delivery vehicles for optical imaging and anticancer therapy were demonstrated in vitro using HT-29 human colon adenocarcinoma cells.

Summary of Recent Advances Nanoparticle-based therapeutics have received increasing attention, as these systems can alleviate many drawbacks of conventional therapy. Metal-organic frameworks (MOFs), a new class of hybrid materials composed of metal ions and organic bridging ligands, have emerged as a promising platform for drug delivery, owing to their high drug loadings, biodegradability, and versatile functionality. The bulk MOF materials can absorb and release large amounts of therapeutics including ibuprofen, procainamide, and nitric oxide. Scale-down of MOFs to the nano-regime yields nanoscale metal-organic frameworks (NMOFs) which are more applicable as delivery vehicles, such as selective delivery of cisplatin prodrugs. Although progress has been made in utilizing NMOFs for drug delivery, many improvements must occur before they can become viable nanotherapeutics.

A PET tracer is desired to help guide the discovery and development of disease-modifying therapeutics for neurodegenerative diseases characterized by neurofibrillary tangles (NFTs), the predominant tau pathology in Alzheimer disease (AD). We describe the preclinical characterization of the NFT PET tracer 18 F-MK-6240. Methods: In vitro binding studies were conducted with 3 H-MK-6240 in tissue slices and homogenates from cognitively normal and AD human brain donors to evaluate tracer affinity and selectivity for NFTs. Immunohistochemistry for phosphorylated tau was performed on human brain slices for comparison with 3 H-MK-6240 binding patterns on adjacent brain slices. PET studies were performed with 18 F-MK-6240 in monkeys to evaluate tracer kinetics and distribution in the brain. 18 F-MK-6240 monkey PET studies were conducted after dosing with unlabeled MK-6240 to evaluate tracer binding selectivity in vivo. Results: The 3 H-MK-6240 binding pattern was consistent with the distribution of phosphorylated tau in human AD brain slices. 3 H-MK-6240 bound with high affinity to human AD brain cortex homogenates containing abundant NFTs but bound poorly to amyloid plaque-rich, NFT-poor AD brain homogenates. 3 H-MK-6240 showed no displaceable binding in the subcortical regions of human AD brain slices and in the hippocampus/entorhinal cortex of non-AD human brain homogenates. In monkey PET studies, 18 F-MK-6240 displayed rapid and homogeneous distribution in the brain. The 18 F-MK-6240 volume of distribution stabilized rapidly, indicating favorable tracer kinetics. No displaceable binding was observed in self-block studies in rhesus monkeys, which do not natively express NFTs. Moderate defluorination was observed as skull uptake. Conclusion: 18 F-MK-6240 is a promising PET tracer for the in vivo quantification of NFTs in AD patients. Cur rently, the clinical evaluation of disease-modifying therapies for Alzheimer disease (AD) requires large, resourceintensive clinical trials focused on measuring cognitive endpoints, which are highly variable. A biomarker that could be used early in clinical development to build confidence in the ability of a therapeutic mechanism to modify disease progression would provide a valuable bridge to investment in a large efficacy study once adequate pharmacokinetics, safety, and tolerability have been established. Biomarkers currently in use (e.g., volumetric MRI, amyloid plaque PET, cerebrospinal fluid measures of amyloid-b and tau) either do not directly inform on modification of disease pathology (volumetric MRI) or do not correlate strongly enough with cognitive decline to measure therapeutic response (amyloid plaque PET and cerebrospinal fluid measures) (1,2). Therefore, there is an unmet need for sensitive biomarkers that quantify early pathologic changes and correlate closely to disease progression and clinical outcomes.Histologic analysis of brains from human autopsy cases have shown that the density and distribution of neurofibrillary tangles (NFTs) correlate with cognitive decline ...

Hybrid nanomaterials, composed of both inorganic and organic components, have recently been examined as promising platforms for imaging and therapeutic applications. This unique class of nanomaterials can not only retain beneficial features of both the inorganic and organic components, but also provides the ability to systematically tune the properties of the hybrid material through the combination of functional components. This feature article will summarize recent advances in the design and synthesis of hybrid nanomaterials and their applications in biological and biomedical areas. The hybrid nanomaterials to be discussed fall into two main categories, silica based materials and nanoscale metal-organic frameworks. Their applications as imaging contrast agents and nanotherapeutics will be highlighted.