Monthly administration of a novel PTH-collagen binding domain fusion protein is anabolic in mice

Drug Delivery Approaches to Improve Tendon Healing

Tendon injuries disrupt the transmission of forces from muscle to bone, leading to chronic pain, disability, and a large socioeconomic burden. Tendon injuries are prevalent; there are over 300,000 tendon repair procedures a year in the United States to address acute trauma or chronic tendinopathy. Successful restoration of function after tendon injury remains challenging clinically. Despite improvements in surgical and physical therapy techniques, the high complication rate of tendon repair procedures motivates the use of therapeutic interventions to augment healing. While many biological and tissue engineering approaches have attempted to promote scarless tendon healing, there is currently no standard clinical treatment to improve tendon healing. Moreover, the limited efficacy of systemic delivery of several promising therapeutic candidates highlights the need for tendon-specific drug delivery approaches to facilitate translation. This review article will synthesize the current state-of-the-art methods that have been used for tendon-targeted delivery through both systemic and local treatments, highlight emerging technologies used for tissue-specific drug delivery in other tissue systems, and outline future challenges and opportunities to enhance tendon healing through targeted drug delivery.

Targeting strategies for bone diseases: signaling pathways and clinical studies

Since the proposal of Paul Ehrlich's magic bullet concept over 100 years ago, tremendous advances have occurred in targeted therapy. From the initial selective antibody, antitoxin to targeted drug delivery that emerged in the past decades, more precise therapeutic efficacy is realized in specific pathological sites of clinical diseases. As a highly pyknotic mineralized tissue with lessened blood flow, bone is characterized by a complex remodeling and homeostatic regulation mechanism, which makes drug therapy for skeletal diseases more challenging than other tissues. Bone-targeted therapy has been considered a promising therapeutic approach for handling such drawbacks. With the deepening understanding of bone biology, improvements in some established bone-targeted drugs and novel therapeutic targets for drugs and deliveries have emerged on the horizon. In this review, we provide a panoramic summary of recent advances in therapeutic strategies based on bone targeting. We highlight targeting strategies based on bone structure and remodeling biology. For bone-targeted therapeutic agents, in addition to improvements of the classic denosumab, romosozumab, and PTH1R ligands, potential regulation of the remodeling process targeting other key membrane expressions, cellular crosstalk, and gene expression, of all bone cells has been exploited. For bone-targeted drug delivery, different delivery strategies targeting bone matrix, bone marrow, and specific bone cells are summarized with a comparison between different targeting ligands. Ultimately, this review will summarize recent advances in the clinical translation of bone-targeted therapies and provide a perspective on the challenges for the application of bone-targeted therapy in the clinic and future trends in this area.

Dual Targeting Anti-Osteoporotic Therapy through Potential Nanotherapeutic Approaches

Osteoporosis is characterised by a major public health burden, particularly taking into account the ageing global population. Therapeutic modalities for osteoporosis are categorised on the basis of their effect on bone remodeling: antiresorptive agents and anabolic agents. Anabolic drugs are favoured as they promote the formation of new bone, whereas antiresorptive drugs terminate the further deterioration of bone. Non-specific delivery of anabolic agents results in prolonged kidney exposure causing malignant hypercalcemia, whereas antiresorptive agents and bisphosphonates may produce osteonecrosis of the jaw. Several clinical trials have been reported for combinational therapy of anabolic agents and antiresorptive agents for osteoporosis. However, none of them have proven their cumulative effectiveness in the treatment of disease. The present work emphasizes on dual-targeting drug delivery approach comprising of bone anabolic and antiresorptive agents that would deliver the therapeutic agents to both the zones of bone simultaneously. The anticipated pioneering delivery approach will intensify the explicit interaction between the therapeutic agent and bone surfaces separately without developing severe adverse effects and improve the osteoporotic therapy effectively compared to non-targeted drug delivery.

Novel bone-targeted parathyroid hormone-related peptide antagonists inhibit breast cancer bone metastases

Patients with advanced breast cancer often develop bone metastases. Treatment is limited to palliative care. Parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP) antagonists for bone metastases failed clinically due to short half-life and inadequate concentration in bone. We synthesized two novel PTHrP antagonists fused to an inert bacterial collagen binding domain (CBD) that directs drugs to bone. PTH(7-33)-CBD is an N-terminal truncated PTHrP antagonist. [W2]PTH(1-33)-CBD is an PTHrP inverse-agonist. The aim of this study was to assess PTH(7-33)-CBD to reduce breast cancer bone metastases and prevent osteolytic destruction in mice and to assess both drugs for apoptosis of breast cancer cells in vitro and inhibition of PTH receptor (PTHR1). PTH(7-33)-CBD (1000 µg/kg, subcutaneous) or vehicle was administered 24 h prior to MDA-MB-231 breast cancer cell inoculation into the tibia marrow. Weekly tumor burden and bone density were measured. Pharmacokinetic analysis of PTH(7-33)-CBD in rat serum was evaluated. Drug effect on cAMP accumulation in SaOS-2 osteosarcoma cells and apoptosis of MDA-MB-231 cells was assessed. PTH(7-33)-CBD reduced MDA-MB-231 tumor burden and osteolytic destruction in mice 4-5 weeks post-treatment. PTH(7-33)-CBD (1000 μg/kg i.v. and subcutaneous) in rats was rapidly absorbed with peak concentration 5-min and terminal half-life 3-h. Bioavailability by the subcutaneous route was 43% relative to the i.v. route. PTH(7-33)-CBD was detected only on rat periosteal bone surfaces that stained positive for collagen-1. PTH(7-33)-CBD and [W2]PTH(1-33)-CBD (10-8M) blocked basal and PTH agonist-induced cAMP accumulation in SaOS-2 osteosarcoma cells. Both drugs induced PTHR1-dependent apoptosis of MDA-MB-231 cells in vitro. Novel bone-targeted PTHrP antagonists represent a new paradigm for treatment of breast cancer bone metastases.

Analysis of the bone fracture targeting properties of osteotropic ligands

PURPOSE

Although more than 18,000,000 fractures occur each year in the US, methods to promote fracture healing still rely primarily on fracture stabilization, with use of bone anabolic agents to accelerate fracture repair limited to rare occasions when the agent can be applied to the fracture surface. Because management of broken bones could be improved if bone anabolic agents could be continuously applied to a fracture over the entire course of the healing process, we undertook to identify strategies that would allow selective concentration of bone anabolic agents on a fracture surface following systemic administration. Moreover, because hydroxyapatite is uniquely exposed on a broken bone, we searched for molecules that would bind with high affinity and specificity for hydroxyapatite. We envisioned that by conjugating such osteotropic ligands to a bone anabolic agent, we could acquire the ability to continuously stimulate fracture healing.

RESULTS

Although bisphosphonates and tetracyclines were capable of localizing small amounts of peptidic payloads to fracture surfaces 2-fold over healthy bone, their specificities and capacities for drug delivery were significantly inferior to subsequent other ligands, and were therefore considered no further. In contrast, short oligopeptides of acidic amino acids were found to localize a peptide payload to a bone fracture 91.9 times more than the control untargeted peptide payload. Furthermore acidic oligopeptides were observed to be capable of targeting all classes of peptides, including hydrophobic, neutral, cationic, anionic, short oligopeptides, and long polypeptides. We further found that highly specific bone fracture targeting of multiple peptidic cargoes can be achieved by subcutaneous injection of the construct.

CONCLUSIONS

Using similar constructs, we anticipate that healing of bone fractures in humans that have relied on immobilization alone can be greately enhanced by continuous stimulation of bone growth using systemic administration of fracture-targeted bone anabolic agents.

Ca2+ -induced orientation of tandem collagen binding domains from clostridial collagenase ColG permits two opposing functions of collagen fibril formation and retardation

To penetrate host tissues, histotoxic clostridia secrete virulence factors including enzymes to hydrolyze extracellular matrix. Clostridium histolyticum, recently renamed as Hathewaya histolytica, produces two classes of collagenase (ColG and ColH). The high-speed AFM study showed that ColG collagenase moves unidirectionally to plane collagen fibril and rebundles fibril when stalled . The structural explanation of the roles for the tandem collagen-binding segment (CBDs) is illuminated by its calcium-bound crystal structure at 1.9 Å resolution (R_work = 15.0%; R_free = 19.6%). Activation may involve calcium-dependent domain rearrangement supported by both small-angle X-ray scattering and size exclusion chromatography. At pCa ≥ 5 (pCa = -log[Ca²⁺ ]), the tandem CBD adopts an extended conformation that may facilitate secretion from the bacterium. At pCa ≤ 4, the compact structure seen in the crystal structure is adopted. This arrangement positions the two binding surfaces ~ 55 Å apart, and possibly ushers ColG along tropocollagen molecules that allow for unidirectional movement. A sequential binding mode where tighter binding CBD2 binds first could aid in processivity as well. Switch from processive collagenolysis to fibril rearrangement could be concentration dependent. Collagen fibril formation is retarded at 1 : 1 molar ratio of tandem CBD to collagen. Tandem CBD may help isolate a tropocollagen molecule from a fibril at this ratio. At 0.1 : 1 to 0.5 : 1 molar ratios fibril self-assembly was accelerated. Gain of function as a result of gene duplication of CBD for the M9B enzymes is speculated. The binding and activation modes described here will aid in drug delivery design.

ACCESSION CODES

The full atomic coordinates of the tandem CBD and its corresponding structure factor amplitudes have been deposited in the Protein Data Bank (PDB accession code 5IKU). Small-angle X-ray scattering data and corresponding ab initio models have been submitted to the Small Angle Scattering Biological Data Bank (SASBDB). Accession codes CL2, collagenase module 2, CN2, CP2 are assigned to envelopes for tandem CBD at -log[Ca²⁺ ] (pCa) 3, 4, 5, and 6, respectively. Accession code DC64 was assigned to the complex of polycystic kidney disease-CBD1-CBD2 with mini-collagen.

Design and Use of Chimeric Proteins Containing a Collagen-Binding Domain for Wound Healing and Bone Regeneration

Collagen-based biomaterials are widely used in the field of tissue engineering; they can be loaded with biomolecules such as growth factors (GFs) to modulate the biological response of the host and thus improve its potential for regeneration. Recombinant chimeric GFs fused to a collagen-binding domain (CBD) have been reported to improve their bioavailability and the host response, especially when combined with an appropriate collagen-based biomaterial. This review first provides an extensive description of the various CBDs that have been fused to proteins, with a focus on the need for accurate characterization of their interaction with collagen. The second part of the review highlights the benefits of various CBD/GF fusion proteins that have been designed for wound healing and bone regeneration.

Alternate carbohydrate and nontraditional inducer leads to increased productivity of a collagen binding domain fusion protein via fed-batch fermentation

The production of collagen binding domain fusion proteins is of significant importance because of their potential as therapeutic biomaterials. It was previously reported that the expression of collagen-binding domain fusion proteins in Escherichia coli was higher when expressed using lactose as an inducer and chemically defined growth media on a shake flask scale. In an effort to further investigate factors that affect expression levels on a fed-batch scale, alternative induction techniques were tested in conjunction with fed-batch fermentation. In this paper, we discuss ten fed-batch fermentation experiments utilizing either glucose or glycerol feed and using lactose or isopropyl-β-d-thiogalactopyranoside (IPTG) as an induction source. It was found that glycerol-fed fermentations induced with lactose allowed for greater expression of target protein, though lesser cell densities were achieved.

Expression of a collagen-binding domain fusion protein: effect of amino acid supplementation, inducer type, and culture conditions

Collagen binding domain fusion proteins are of significant importance because of their potential as therapeutic biomaterials. In this paper, we investigate the production of such therapeutic proteins via fermentation of Escherichia coli on both an undefined medium and a defined medium. Defined media with amino acid supplementation provided higher amounts of therapeutic protein than undefined media with no supplementation. Additionally, utilizing lactose instead of isopropyl-β-d-thio-galactoside (IPTG) for induction and extending batch time yielded higher amounts of the model therapeutic.

Structures of three polycystic kidney disease-like domains from Clostridium histolyticum collagenases ColG and ColH

The surface properties and dynamics of PKD-like domains from ColG and ColH differ.

Clostridium histolyticum collagenases ColG and ColH are segmental enzymes that are thought to be activated by Ca²⁺-triggered domain reorientation to cause extensive tissue destruction. The collagenases consist of a collagenase module (s1), a variable number of polycystic kidney disease-like (PKD-like) domains (s2a and s2b in ColH and s2 in ColG) and a variable number of collagen-binding domains (s3 in ColH and s3a and s3b in ColG). The X-ray crystal structures of Ca²⁺-bound holo s2b (1.4 Å resolution, R = 15.0%, R_free = 19.1%) and holo s2a (1.9 Å resolution, R = 16.3%, R_free = 20.7%), as well as of Ca²⁺-free apo s2a (1.8 Å resolution, R = 20.7%, R_free = 27.2%) and two new forms of N-terminally truncated apo s2 (1.4 Å resolution, R = 16.9%, R_free = 21.2%; 1.6 Å resolution, R = 16.2%, R_free = 19.2%), are reported. The structurally similar PKD-like domains resemble the V-set Ig fold. In addition to a conserved β-bulge, the PKD-like domains feature a second bulge that also changes the allegiance of the subsequent β-strand. This β-bulge and the genesis of a Ca²⁺ pocket in the archaeal PKD-like domain suggest a close kinship between bacterial and archaeal PKD-like domains. Different surface properties and indications of different dynamics suggest unique roles for the PKD-like domains in ColG and in ColH. Surface aromatic residues found on ColH s2a-s2b, but not on ColG s2, may provide the weak interaction in the biphasic collagen-binding mode previously found in s2b-s3. B-factor analyses suggest that in the presence of Ca²⁺ the midsection of s2 becomes more flexible but the midsections of s2a and s2b stay rigid. The different surface properties and dynamics of the domains suggest that the PKD-like domains of M9B bacterial collagenase can be grouped into either a ColG subset or a ColH subset. The conserved properties of PKD-like domains in ColG and in ColH include Ca²⁺ binding. Conserved residues not only interact with Ca²⁺, but also position the Ca²⁺-interacting water molecule. Ca²⁺ aligns the N-terminal linker approximately parallel to the major axis of the domain. Ca²⁺ binding also increases stability against heat and guanidine hydrochloride, and may improve the longevity in the extracellular matrix. The results of this study will further assist in developing collagen-targeting vehicles for various signal molecules.

Pharmacokinetics in rats of a long-acting human parathyroid hormone-collagen binding domain peptide construct

The pharmacokinetics of a hybrid peptide consisting of the N-terminal biologically active region of human parathyroid hormone (PTH) linked to a collagen-binding domain (CBD) were evaluated in female Sprague-Dawley rats. The peptide, PTH-CBD, consists of the first 33 amino acids of PTH linked as an extension of the amino acid chain to the CBD peptide derived from ColH collagenase of Clostridium histolyticum. Serum concentrations arising from single dose administration by the subcutaneous and intravenous routes were compared with those measured following route-specific mole equivalent doses of PTH(1-34). Population-based modeling demonstrated similar systemic absorption kinetics and bioavailability for both peptides. Exposure to PTH-CBD was sixfold higher because of a systemic clearance of approximately 20% relative to PTH(1-34); however, these kinetics were consistent with more than 95% of a dose being eliminated from serum within 24 h. Results obtained support continued investigation of PTH-CBD as a bone-targeted anabolic agent for the treatment of postmenopausal osteoporosis.

Treating osteoporosis by targeting parathyroid hormone to bone

Osteoporosis is a major public health problem despite widespread use of bisphosphonate therapy. PTH(1-34) is a more effective treatment; but its use has been limited by side effects (hypercalcemia, tumor risk) and inconvenient dosing (daily injection). Long-acting forms of PTH are also effective but cause severe hypercalcemia, presumably from effects in kidney. We hypothesized that targeted delivery of PTH to bone using a collagen binding domain (PTH-CBD) could reduce hypercalcemia. PTH-CBD is cleared from serum within 12hours after subcutaneous administration. In ovariectomized rats, monthly administration of PTH-CBD increased spinal BMD by 14.2% with no associated hypercalcemia. Such bone-targeted anabolic agents may ultimately allow the superior efficacy of anabolic therapy to be obtained with the dosing convenience of bisphosphonates.

Structural comparison of ColH and ColG collagen-binding domains from Clostridium histolyticum

Clostridium histolyticum secretes collagenases, ColG and ColH, that cause extensive tissue destruction in myonecrosis. The C-terminal collagen-binding domain (CBD) of collagenase is required for insoluble collagen fibril binding and subsequent collagenolysis. The high-resolution crystal structures of ColG-CBD (s3b) and ColH-CBD (s3) are reported in this paper. The new X-ray structure of s3 was solved at 2.0-Å resolution (R = 17.4%; R(free) = 23.3%), while the resolution of the previously determined s3b was extended to 1.4 Å (R = 17.9%; R(free) = 21.0%). Despite sharing only 30% sequence identity, the molecules resemble one another closely (root mean square deviation [RMSD] C(α) = 1.5 Å). All but one residue, whose side chain chelates with Ca(2+), are conserved. The dual Ca(2+) binding site in s3 is completed by an unconserved aspartate. Differential scanning calorimetric measurements showed that s3 gains thermal stability, comparable to s3b, by binding to Ca(2+) (holo T(m) = 94.1°C; apo T(m) = 70.2°C). holo s3 is also stabilized against chemical denaturants urea and guanidine HCl. The three most critical residues for collagen interaction in s3b are conserved in s3. The general shape of the binding pocket is retained by altered loop structures and side chain positions. Small-angle X-ray scattering data revealed that s3 also binds asymmetrically to minicollagen. Besides the calcium-binding sites and the collagen-binding pocket, architecturally important hydrophobic residues and the hydrogen-bonding network around the cis-peptide bond are well conserved within the metallopeptidase subfamily M9B. CBDs were previously shown to bind to the extracellular matrix of various tissues. Compactness and extreme stability in physiological Ca(2+) concentration possibly make both CBDs suitable for targeted growth factor delivery.

Bacterial collagen-binding domain targets undertwisted regions of collagen

Clostridium histolyticum collagenase causes extensive degradation of collagen in connective tissue that results in gas gangrene. The C-terminal collagen-binding domain (CBD) of these enzymes is the minimal segment required to bind to a collagen fibril. CBD binds unidirectionally to the undertwisted C-terminus of triple helical collagen. Here, we examine whether CBD could also target undertwisted regions even in the middle of the triple helix. Collageneous peptides with an additional undertwisted region were synthesized by introducing a Gly → Ala substitution [(POG)(x) POA(POG)(y)]₃, where x + y = 9 and x > 3). ¹H-¹⁵N heteronuclear single quantum coherence nuclear magnetic resonance (HSQC NMR) titration studies with ¹⁵N-labeled CBD demonstrated that the minicollagen binds to a 10 Å wide 25 Å long cleft. Six collagenous peptides each labeled with a nitroxide radical were then titrated with ¹⁵N-labeled CBD. CBD binds to either the Gly → Ala substitution site or to the C-terminus of each minicollagen. Small-angle X-ray scattering measurements revealed that CBD prefers to bind the Gly → Ala site to the C-terminus. The HSQC NMR spectra of ¹⁵N-labeled minicollagen and minicollagen with undertwisted regions were unaffected by the titration of unlabeled CBD. The results imply that CBD binds to the undertwisted region of the minicollagen but does not actively unwind the triple helix.

A single injection of the anabolic bone agent, parathyroid hormone-collagen binding domain (PTH-CBD), results in sustained increases in bone mineral density for up to 12 months in normal female mice

Parathyroid hormone (PTH) is the most effective osteoporosis treatment, but it is only effective if administered by daily injections. We fused PTH(1-33) to a collagen binding domain (PTH-CBD) to extend its activity, and have shown an anabolic bone effect with monthly dosing. We tested the duration of action of this compound with different routes of administration. Normal young C57BL/6J mice received a single intraperitoneal injection of PTH-CBD (320 μg/kg). PTH-CBD treated mice showed a 22.2 % increase in bone mineral density (BMD) at 6 months and 12.8 % increase at 12 months. When administered by subcutaneous injection, PTH-CBD again caused increases in BMD, 15.2 % at 6 months and 14.3 % at 12 months. Radiolabeled PTH-CBD was concentrated in bone and skin after either route of administration. We further investigated skin effects of PTH-CBD, and histological analysis revealed an apparent increase in anagen VI hair follicles. A single dose of PTH-CBD caused sustained increases in BMD by >10 % for 1 year in normal mice, regardless of the route of administration, thus showing promise as a potential osteoporosis therapy.

Probing the 3-D structure, dynamics, and stability of bacterial collagenase collagen binding domain (apo- versus holo-) by limited proteolysis MALDI-TOF MS

Pairing limited proteolysis and matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) to probe clostridial collagenase collagen binding domain (CBD) reveals the solution dynamics and stability of the protein, as these factors are crucial to CBD effectiveness as a drug-delivery vehicle. MS analysis of proteolytic digests indicates initial cleavage sites, thereby specifying the less stable and highly accessible regions of CBD. Modulation of protein structure and stability upon metal binding is shown through MS analysis of calcium-bound and cobalt-bound CBD proteolytic digests. Previously determined X-ray crystal structures illustrate that calcium binding induces secondary structure transformation in the highly mobile N-terminal arm and increases protein stability. MS-based detection of exposed residues confirms protein flexibility, accentuates N-terminal dynamics, and demonstrates increased global protein stability exported by calcium binding. Additionally, apo- and calcium-bound CBD proteolysis sites correlate well with crystallographic B-factors, accessibility, and enzyme specificity. MS-observed cleavage sites with no clear correlations are explained either by crystal contacts of the X-ray crystal structures or by observed differences between Molecules A and B in the X-ray crystal structures. The study newly reveals the absence of the βA strand and thus the very dynamic N-terminal linker, as corroborated by the solution X-ray scattering results. Cobalt binding has a regional effect on the solution phase stability of CBD, as limited proteolysis data implies the capture of an intermediate-CBD solution structure when cobalt is bound.

Prevention of chemotherapy-induced osteoporosis by cyclophosphamide with a long-acting form of parathyroid hormone

BACKGROUND

Most chemotherapeutics reduce bone mineral density (BMD) and increase risk for fractures by causing gonadal suppression, which in turn increases bone removal. Cyclophosphamide (CYP) also has a direct effect of inhibiting bone formation and removal, making the resulting bone loss particularly difficult to treat with antiresorptive therapy.

AIM

We tested whether a single dose of the anabolic agent PTH linked to a collagen binding domain (PTHCBD) could prevent the effects of CYP-induced bone loss.

METHODS

Mice received either buffer alone, CYP, or CYP+ PTH-CBD. BMD and alkaline phosphatase were measured every 2 weeks for a total of 8 weeks.

RESULTS

After 6 weeks, mice treated with CYP showed expected reductions in BMD (increase from baseline: 7.4 ± 6.9 vs 24.35 ± 4.86% in mice without chemotherapy, p<0.05) and decrease in alkaline phosphatase levels (42.78 ± 6.06 vs 60.62 ± 6.23 IU/l in mice without chemotherapy, p<0.05), consistent with osteoporosis from impaired bone formation. Administration of a single dose of PTH-CBD (320 μg/kg ip) prior to CYP treatment improved BMD (change from baseline: 23.4 ± 5.4 vs 7.4 ± 6.9%, CYP treatment alone, p<0.05) and increased alkaline phosphatase levels (50.14 ± 4.86 vs 42.78 ± 6.06 IU/l in CYP treatment alone, p<0.05). BMD values and alkaline phosphatase levels were restored to those seen in mice not receiving chemotherapy.

CONCLUSIONS

A single dose of PTHCBD prior to chemotherapy reversed CYP-induced suppression of bone formation and prevented CYP-induced bone loss in mice.

Bone targeting for the treatment of osteoporosis

Osteoporosis represents a major public health burden especially considering the aging populations worldwide. Drug targeting will be important to better meet these challenges and direct the full therapeutic potential of therapeutics to their intended site of action. This review has been organized in modules, such that scientists working in the field can easily gain specific insight in the field of bone targeting for the drug class they are interested in. We review currently approved and emerging treatment options for osteoporosis and discuss these in light of the benefit these would gain from advanced targeting. In addition, established targeting strategies are reviewed and novel opportunities as well as promising areas are presented along with pharmaceutical strategies how to render novel composites consisting of a drug and a targeting moiety responsive to bone-specific or disease-specific environmental stimuli. Successful implementation of these principles into drug development programs for osteoporosis will substantially contribute to the clinical success of anti-catabolic and anabolic drugs of the future.