Due to their large size, charged surfaces, and environmental sensitivity, proteins do not naturally cross cell-membranes in intact form and, therefore, are difficult to deliver for both diagnostic and therapeutic purposes. Based upon the observation that clustered oligonucleotides can naturally engage scavenger receptors that facilitate cellular transfection, nucleic acid–metal organic framework nanoparticle (MOF NP) conjugates have been designed and synthesized from NU-1000 and PCN-222/MOF-545, respectively, and phosphate-terminated oligonucleotides. They have been characterized structurally and with respect to their ability to enter mammalian cells. The MOFs act as protein hosts, and their densely functionalized, oligonucleotide-rich surfaces make them colloidally stable and ensure facile cellular entry. With insulin as a model protein, high loading and a 10-fold enhancement of cellular uptake (as compared to that of the native protein) were achieved. Importantly, this approach can be generalized to facilitate the delivery of a variety of proteins as biological probes or potential therapeutics.
Proteins play key roles in living systems, and the ability to deliver active proteins to cells is attractive for both diagnostic and therapeutic purposes. Potential uses involve the evaluation of metabolic pathways, regulation of cellular processes, and treatment of disease involving protein deficiencies. (4−6) During the past decade, a series of techniques have been developed to facilitate protein internalization by live cells, including the use of complementary transfection agents, nanocarriers, (7−9) and protein surface modifications. (10−13) Although each strategy has its own merit, none are perfect solutions; they can cause cytotoxicity, reduce protein activity, and suffer from low delivery payloads. (14) For example, we have made the observation that one can take almost any protein and functionalize its surface with DNA to create entities that will naturally engage the cell-surface receptors involved in spherical nucleic acid (SNA) uptake. (13,15−17) While this method is extremely useful in certain situations, it requires direct modification of the protein and large amounts of nucleic acid, on a per-protein basis, to effect transfection. Ideally, one would like to deliver intact, functional proteins without the need to chemically modify them, and to do so in a nucleic-acid efficient manner.
Metal organic frameworks (MOFs) have emerged as a class of promising materials for the immobilization and storage of functional proteins. (18) Their mesoporous structures allow for exceptionally high protein loadings, and their framework architectures can significantly improve the thermal and chemical stabilities of the encapsulated proteins. (19−24) However, although MOF NPs have been recognized as potentially important intracellular delivery vehicles for proteins, (25−27) their poor colloidal stability and positively charged surfaces, (28,29) inhibit their cellular uptake and have led to unfavorable bioavailabilities. (30−33) Therefore, the development of general approaches for reducing MOF NP aggregation, minimizing positive charge (which can cause cytotoxicity), and facilitating cellular uptake is desirable. (34,35)
