The most routinely recommended treatments for OA, including orally administered or locally injected anti-inflammatory agents and analgesics, are palliative, with surgical joint replacement the only option in advanced disease [9
]. The only therapies purported to be disease-modifying aim to replenish cartilage proteoglycan components via dietary supplementation with chondroitin sulfate/glucosamine [44
] or via intra-articular injection of hyaluronic acid (e.g. Synvisc) [46
]. However, there is no consensus on the efficacy of oral ingestion of aggrecan sugar moieties [45
], and joint injections of hyaluronic acid demonstrate efficacy at relieving knee joint pain, but only for brief periods of up to 6 months [47
]. Therefore, the development of an effective agent supporting protective and/or regenerative effects in articular cartilage would have an immediate and major impact on standard of care for this pervasive and debilitating disease. Our findings establish teriparatide, in particular the FDA-approved drug Forteo, as a disease-modifying candidate therapeutic that has both chondro-protective and chondro-regenerative capabilities in the context of OA.
At a cellular level, it has been suggested that aberrant maturation of articular chondrocytes along a pathway that resembles endochondral ossification contributes to OA progression [15
]. This inappropriate articular chondrocyte differentiation is associated with the up-regulation of Runx2 [48
], type × collagen [12
], and MMP13 [43
], as well as increased apoptosis [14
] and other hallmarks of chondrocyte maturation [15
], in both human OA and animal models of disease. Consistent with this, in our mouse model of MLI-induced OA, we observed the up-regulation of Adamts5
, COL10a1, RUNX2, MMP13 message and protein and increased degradation of aggrecan in degenerating articular cartilage.
Signaling pathways inducing chondrocyte maturation and OA-like cartilage degeneration include loss of TGF-β signaling in mice [50
]; gain of WNT/β-CATENIN signaling in mice [53
] and humans [55
]; gain of Indian Hedgehog (IHH) signaling in mice and humans [58
]; and increased HIF2α expression leading to enhanced IHH/RUNX2 signaling in mice [59
]. These pro-maturation shifts in articular chondrocyte signaling are generally consistent with the additional findings that RUNX2 is up-regulated in injury-induced murine knee OA [48
], and over-expression of RUNX2 in articular chondrocytes that are experiencing mechanical stress contributes to the pathogenesis of OA [61
]. These and other similar findings implicate maturation-driving signals in the pathogenesis of OA.
Modulation of signaling pathways that activate chondrocyte maturation might be protective against cartilage degeneration. For example, Runx2
-haploinsufficient mice administered a meniscal injury display less severe knee OA [48
]. Similarly, pharmacologic and genetic inhibition of IHH signaling in mice protects against injury-induced degeneration of knee cartilage that is concomitant with down-regulation of Runx2
]. Furthermore, inhibition of cartilage-degrading enzymes associated with chondrocyte maturation, such as MMP13 and ADAMTS5, has been underscored as the best current strategic direction for developing an OA therapeutic [62
]. Thus, inhibition of inappropriate articular chondrocyte maturation and/or blockade of the associated matrix-degrading enzymes represent obvious targets for developing a treatment for OA.
It is well-established that parathyroid hormone and PTHrP are potent inhibitors of chondrocyte maturation [22
]. Specifically, activation of PTHR1 potently induces chondrocyte proliferation and matrix production (type II collagen and proteoglycans) while suppressing maturation [23
]. This concept is supported by in vitro
and in vivo
data, where gain or loss of PTHR1 signaling respectively inhibits or accelerates chondrocyte hypertrophy [30
]. Owing to selective up-regulation of PTHR1, which we identified in human cartilage following meniscal injury or in progressive OA and in mouse cartilage following MLI, PTHR1 signaling may be protective and possibly even regenerative in the context of cartilage degeneration. This idea is supported by two studies which demonstrate that when administered intermittently via intra-articular injection, PTH can decelerate papain-induced cartilage degeneration in the rat [63
] and can induce cartilage regeneration in a full-thickness osteochondral injury in the rabbit [64
]. Although these studies suggest that PTH has chondro-regenerative potential, the models of cartilage degeneration and injury that were employed do not examine the effect of PTH during the OA disease process. Additionally, the daily intra-articular injection treatment regimen used in these studies is not clinically practical or translational, leaving open the need to examine the efficacy of the systemic mode of delivery that is currently FDA-approved for teriparatide. Therefore, the rationale for the present investigation of teriparatide as a potential systemic OA therapy was based on two key concepts: The broad literature, which establishes PTHR1 signaling in chondrocytes as an inducer of matrix production and inhibitor of maturation, and the selective up-regulation of PTHR1 after injury and in arthritic cartilage, which primes the cells to be targeted by teriparatide therapy.
The experimental design to test teriparatide as an OA therapy involved treatment of mice administered a meniscal/ligamentous knee injury followed by treatment with either Forteo or PTH at the 40 μg/kg/day dose. This dose was selected based on literature establishing that the effective teriparatide dose range for treatment of bone loss or fracture repair in mice is between 30 μg/kg/day and 400 μg/kg/day [16
]. In two of these studies, 40 μg/kg/day was demonstrated to effectively accelerate fracture healing in the mouse, with enhancement of chondrogenesis and expansion of the cartilaginous callus [65
]. It should be noted that 40μg/kg/day is significantly higher than accepted and optimal ranges in other species including rat (4–20μg/kg/day) [69
], rabbit (10 μg/kg/day) [71
], macaque (5 μg/kg/day) [72
], and human (0.25 μg/kg/day) [73
]. We anticipate that there is a similar species-associated shift in the range of effective concentrations supporting chondro-protective and chondro-regenerative effects.
Compared to saline treatment and when delivered immediately following injury, Forteo was chondro-protective, characterized by enhanced proteoglycan production by 4 weeks and decelerated cartilage degeneration at 12 weeks. Notably, the more clinically-relevant delayed treatment regimen elicited a chondro-regenerative and maturation-inhibiting effect, characterized by increased proteoglycan content in the articular cartilage, up-regulation of Prg4, an increased amount of articular cartilage, and decreased articular chondrocyte expression of COLXa1, RUNX2, and MMP13. Degeneration of aggrecan was also reduced. Chondro-regenerative effects were also observed when injured mice were treated with PTH, excluding the possibility that the effects are unique to the Forteo formulation. Ameliorating the concern that teriparatide might exacerbate osteophyte formation in degenerating joints, microCT analyses and histological measurement of osteophyte number and diameter establish that osteophytes were not increased in injured joints from Forteo-treated mice.
Histological and histomorphometric data suggest that inhibition of matrix-degrading enzymes may be more effective then teriparatide therapy at stopping degeneration of cartilage matrix during OA (74
). Specifically, compared to joints from injured mice treated with Forteo or PTH in the present study, the articular cartilage is better preserved following injury-induction of OA in Adamts5
] and Mmp13
] knockout mice. However, the genetic ablation of catabolic enzymes in these studies leads to complete arrest of matrix degeneration without affecting other changes associated with disease, including aberrant chondrocyte maturation, osteophyte formation and subchondral sclerosis. This is in contrast to the chondro-regeneration and inhibition of chondrocyte maturation seen in the present study following teriparatide administration, which suggests this therapy may be effective at decelerating several key disease phenotypes in addition to cartilage degeneration. Nevertheless, the chondro-preservation observed in the Adamts5
knockout mice has led to the conclusion that inhibitory agents targeting these enzymes have remarkable therapeutic potential [62
]. Initial studies in the rat have substantiated the in vivo
efficacy of an aggrecanase inhibitor in the blockade of aggrecan degradation [76
] and MMP inhibitors in decelerating cartilage degeneration [77
]. The only way to elicit similar cartilage preservation clinically would be to administer enzyme inhibitors before degeneration begins, and to persist with the treatment over the span of a lifetime. To begin treatment when a patient presents to the clinic with pain due to progressed disease would potentially prevent further degeneration, but it would not have a reparative effect that would build back matrix. A potential strategy for more effective therapeutic results would be to combine periodic teriparatide treatment with the administration of an ADAMTS or MMP inhibitor; this could lead to even more robust chondro-regeneration by teriparatide owing to the complementary blockade of matrix degradation.
Although teriparatide elicits a bone anabolic effect in the subchondral plate via activation of osteoblasts, it might simultaneously restrict the endochondral ossification-like process occurring in MLI-associated osteophytes. This would occur via inhibition of chondrocyte maturation in a manner analogous to the action of PTHrP in the developing growth plate. This possibility is supported by our findings that Forteo inhibits expression of genes associated with chondrocyte maturation in degenerating cartilage. Regarding the increased bone volume in the subchondral plate in both MLI and sham-operated joints following Forteo treatment, there is the concern that this effect might enhance subchondral sclerosis and exacerbate, rather than decelerate, the cartilage degeneration that occurs in clinical OA. This concern, coupled with practical considerations regarding the persistence of the chondro-protective or -regenerative effects of teriparatide after the treatment is stopped, represent key questions to be addressed.
There is evidence for teriparatide induction of osteosarcoma in rats administered long term daily treatment out to 2 years, with a 20–40% incidence at 70–80% of life span [79
]. In our examination of articular cartilage in samples harvested from teriparatide-treated mice, we never observed a lesion consistent with a malignancy in the tibial or femoral metaphyses. Because these lesions are among the most common sites for primary osteosarcoma, we do not believe that our immediate or delayed treatment regimens are carcinogenic in the mouse. This is consistent with what has been reported in macaques, where teriparatide did not induce any osteosarcomas following long term treatment with 5 μg/kg/day [72
]. Furthermore, human data suggest only 2 cases in >430,000 osteoporosis patients treated with Forteo developed osteosarcoma [73
]. Thus, we believe that the increase in osteosarcoma incidence in teriparatide-treated rats probably is not prognostic of an equivalent risk in humans [80
], and is possibly species-specific.
In conclusion, we have identified teriparatide as a novel candidate therapy for injury-induced OA that is both chondro-protective and chondro-regenerative. This impacts the arthritis field by providing the basis for FDA-approved Forteo as a disease-modifying therapy for OA with clinical potential. Overall, given the scope of the clinical problem and the current availability of Forteo for clinical use, our experimental findings make a compelling case for further characterization of this drug in a clinical trial aimed at evaluating its efficacy as a treatment for human OA.