Technology

Bone Targeting

Selective drug delivery to target tissues can be advantageous as it can help reduce both drug dosage and side effects in other tissues. This strategy has been pioneered in cancer therapy where it has proven to be particularly useful for the specific delivery of toxic drugs to tumors.

Bone diseases could also benefit from the development of a general strategy for selectively targeting drugs to bone. Hydroxyapatite (HA) is a major inorganic component of hard tissues (such as bone), and it does not exist in soft tissues. Thus targeting a drug onto HA could be a promising way for selective drug delivery to bone.

Enobia’s bone-targeting strategy for proteins and small molecules uses acidic peptide sequences such as poly-aspartic or poly-glutamic acids conjugation. This acidic bone-targeting motif is found in several proteins such as osteopontin and bone sialoprotein, where their binding to the bone matrix is required. Because repeated peptide sequences of aspartic and glutamic acids are made of natural amino acid sequences recombinant DNA technology is used to directly synthesize bone targeted proteins such as enzymes.

Enobia has a range of patents providing broad coverage for this bone-targeting technology.

Bone-targeting efficacy of this approach was demonstrated with the enzyme gluthatione S-transferase (GST) fused to a repetitive sequence of 10 aspartic acid residues and produced in the E. coli bacterial system. This fusion protein was called GST-D10 in reference to its C-terminal extension of 10 aspartic acid residues. A sample of the purified GST-D10 preparation was then incubated with a sample of reconstituted bone mineral phase in suspension and the amount of protein bound to the mineral phase was measured. Fusion of the D10 sequence to GST caused a 6 fold increase in binding to the mineral phase of bone, compared to non modified GST.



The distribution of GST-D10 to bone compared to non-bone targeted GST, was also assessed after a bolus administration to mice. Results demonstrate that fusion of D10 to GST caused preferential accumulation in bone while having not significant impact on the distribution in serum.

This bone targeting technology was applied to two enzymes that Enobia is using in the development of enzyme replacement therapy for X-linked hypophosphatemia (PHEX) and hypophosphatasia (alkaline phosphatase (ALP)).

D10 increases binding of recombinant sPHEX and sALP to hydroxyapatite in vitro.

When bone targeted enzyme (alkaline phosphatase) is administered to mice, the enzyme preferentially accumulates in bone (tibia, femur, calvaria) compared to other tissue such as serum and muscle.

When bone targeted enzyme (alkaline phosphatase) is administered to mice, the enzyme preferentially accumulates in bone (tibia, femur, calvaria) compared to other tissue such as serum and muscle.