Translational In Vivo Models and Assays for Drug Discovery
Drug discovery for osteoarthritis (OA) is crucial due to the significant impact of the disease on joint function, mobility, pain, and quality of life, affecting a large portion of the population. Currently, there are limited effective pharmacological interventions, emphasizing the urgent need for novel therapeutic options to alleviate symptoms and modify the course of the disease. Advancements in drug discovery not only address the unmet medical needs in managing OA but also contribute to reducing the socioeconomic burden associated with this prevalent musculoskeletal condition.
The medial meniscal tear (MMT) OA model involves cutting the medial meniscus so that the meniscus no longer protects the medial surfaces of the tibial plateau and femoral condyle, mimicking a common injury seen in humans. Mice, rats, and guinea pigs are used to create an MMT OA model, with rats being the most utilized species. However, a benefit to using guinea pigs for the MMT OA model is that they will develop spontaneous OA. Thus, the contralateral non-operated joint in a guinea pig MMT OA model can be utilized in the evaluation of effects of various treatments.
The induced meniscal tear in the MMT OA model leads to altered biomechanics and increased stress on the articular cartilage within the joint causing progressive cartilage degeneration. By 3- to 6-weeks post-surgery, tibial cartilage degeneration may be focally severe on the outer one-third of the tibia (and less severe in the middle and inner areas). Additionally, osteophytes form and progressively increase in size, and substantial subchondral bone changes occur in the medial tibia beneath the cartilage lesions, allowing assessment of both chondroprotective and bone effects. Of the major microsurgery models, the MMT OA model has been shown to be a more reliable model than the anterior cruciate ligament (ACL) transection-induced arthritis model, which induces excessively severe lesions with much greater variability.
Representative photomicrographs of knee joint sections stained with toluidine blue from rat MMT OA models (right column) 28-days (top row) and 63-days (bottom row) post-surgery. Stained sections of knee joints from age-matched, normal rats (left column) shown for comparison to illustrate disease progression.
Representative photomicrographs of knee joint sections, stained with toluidine blue, from a normal rat (A) and a rat MMT OA model (B; 28-days post-surgery), depicting the division of the tibial plateau into 3 zones of equal width using an ocular micrometer, with zone 1 on the outside and zone 3 on the inside. (C) Section of a knee joint from a rat MMT OA model, stained with toluidine blue, depicting substantial cartilage degeneration scoring.
Destabilization of the medial meniscus (DMM) OA model involves inducing structural changes in the knee joint by surgically cutting the medial meniscotibial ligament, leading to destabilization of the medial meniscus. Conducted in either mice or rats, this manipulation induces mild to moderate OA lesions in the central weight bearing area of the medial femoral condyle and medial tibial plateau. Lesions are evident as early as 2-weeks post-surgery and increase in severity over time at a slower rate than in the MMT model. Of the major microsurgery models, the DMM OA model has been shown to be a more reliable model than the ACL transection-induced arthritis model, which induces excessively severe lesions with much greater variability.
Representative photomicrographs of knee joint sections stained with toluidine blue from a rat MMT OA model 28-days post-surgery (A) and from a rat DMM OA model 56-days post-surgery (B).
Mean cartilage degeneration scores for three equally spaced regions across the medial tibial plateau surface, Zone 1 (outside region, red bars), Zone 2 (middle region, orange bars), Zone 3 (inside region, light orange bars), along with the sum of the three areas (grey bars), for a rat MMT OA model 28-days post-surgery (A) and rat DMM OA model 56-days post-surgery (B).
The monoiodoacetate (MIA)-induced OA model involves injecting MIA, an inhibitor of glycolysis, into a joint, typically the knee, to kill chondrocytes, leading to an acellular matrix and eventual collapse of the cartilage matrix into the epiphysis. This model is biphasic as it simulates early joint pain due to inflammation, and then severe cartilage degradation and bone changes, which is seen in humans with end-stage OA. This model has diverse applications. Researchers employ this model to study the pathogenesis of OA pain and gain insights into the underlying mechanisms of joint degeneration. Additionally, this model can be used to assess agents designed to hinder acute proteoglycan loss via aggrecanase or MMPs, impede collagen matrix degeneration through collagenase, and induce repair. It is also valuable for evaluating the impact of agents on bone remodeling given its propensity to result in sclerotic subchondral bone lesions and osteophyte formation. Additionally, there is substantial arthrofibrosis, making this model a good tool for evaluating methods to inhibit this process.
Rat knee joints, stained with toluidine blue, from rats injected intra-articularly with saline (A), 0.5 mg of MIA (B) or 2.0 mg of MIA (C). The lower dose of MIA results in milder, more variable lesions whereas the higher dose of MIA results in severe, more consistent lesions.
Weight bearing asymmetry, a translatable measure of joint pain, was assessed in a rat model of MIA-induced OA (red boxes) and control animals (grey boxes). Rats treated with MIA exhibited differences in weight exerted by its hind paws, with the untreated (left) limb wielding more weight than the treated (right) limb. There was little difference in weight exerted by the hind paws of the control rats.
Spontaneous OA manifests in the medial compartment of the knee joint in a variety of guinea pig strains. While occurring in both genders, males exhibit faster growth, reaching higher body weights and displaying more consistent pathological alterations. Typically, the disease is bilaterally symmetrical in terms of incidence and severity. Initial changes, observed around 3-months of age (700 grams weight), involve focal chondrocyte death, proteoglycan loss, and fibrillation in the medial tibial plateau. OA pathogenesis progresses, and by 1-year, profound cartilage degeneration, subchondral sclerosis, bone cysts, and severe meniscal degeneration has occurred. The predictability and similarity to human disease make this guinea pig model valuable for studying OA pathogenesis and potential therapeutic interventions.
Knee joint from 4-month-old guinea pig (A). Photomicrographs of toluidine blue-stained sections of the medial aspect of a knee joint obtained from a 6-month (B), 9.5-month (C) and 18-month-old (D) guinea pig. Images illustrate progression of cartilage degeneration on the tibial plateau.
