The active metabolite of leflunomide, A77 1726, increases the production of IL-1 receptor antagonist in human synovial fibroblasts and articular chondrocytes

Leflunomide is an oral immunomodulatory agent, which is considered effective for the treatment of rheumatoid arthritis (RA). Leflunomide is a disease-modifying antirheumatic drug that is approved for treatment of RA, and radiographical findings indicate that it delays joint damage [1‐4]. Its therapeutic profile closely resembles that of methotrexate. The latter drug is the most widely used disease-modifying antirheumatic drug but, despite a favourable efficiency–toxicity profile, in numerous patients it is either insufficient or associated with unacceptable side effects.

In vivo, leflunomide is rapidly converted into its pharmacologically active metabolite A77 1726 [5]. The recommended dose of leflunomide for the treatment of RA patients is 20 mg/day, which produces steady-state serum levels of A77 1726 of approximately 25–45 μg/ml (75–115 μmol/l) [6]. Although the precise mode of action of leflunomide in vivo remains elusive, A77 1726 has been shown in vitro to inhibit reversibly dihydro-orotate dehydrogenase (DHODH), which catalyzes a rate-limiting step in the de novo synthesis of pyrimidines [7, 8]. The inhibition of DHODH activity by A77 1726 might explain part of its mechanism of action in suppressing inflammation. Indeed, many effects of A77 1726 can be reversed by exposing target cells to the product of DHODH activity, namely uridine. Leflunomide is a potent noncytotoxic inhibitor of the proliferation of stimulated B and T lymphocytes, which depend on de novo pyrimidine synthesis to fulfill their metabolic needs [4, 5]. Furthermore, leflunomide blocks tumour necrosis factor (TNF)-α-mediated cellular responses in T cells by inhibiting nuclear factor-κB – a mechanism that also depends on pyrimidine biosynthesis [9, 10]. In addition, A77 1726 exerts a direct inhibitory effect on cyclo-oxygenase (COX)-2 activity, both in vitro and in vivo [11, 12]. Finally, it has been reported that, at higher concentrations, A77 1726 inhibits different types of receptor and nonreceptor tyrosine kinases that are involved in cytokine and growth factor signalling [13‐15].

RA is characterized by synoviocyte proliferation and infiltration of inflammatory cells, such as lymphocytes and macrophages, into the joint. Local release of proinflammatory mediators and metalloproteinases causes joint cartilage destruction and leads to the perpetuation of joint inflammation. Potential direct anti-inflammatory effects of A77 1726 on joint cells are thus of interest because of their relevance to the effectiveness of leflunomide in treating RA and other cartilage-damaging diseases. In a previous study, A77 1726 was found to inhibit the expression of monocyte-activating factor at the surface of T lymphocytes, which in turn decreased the activation of monocyte/macrophages, and thus their production of IL-1β and matrix metalloproteinase (MMP)-1 [16]. A further study showed that A77 1726 inhibits the production of prostaglandin E₂ (PGE₂), MMP-1 and IL-6 in human synovial fibroblasts [12]. The inhibition of MMP-1 and IL-6 production was due to the well known inhibitory effect of A77 1726 on pyrimidine synthesis, because it was reversed by the addition of uridine. PGE₂ production appeared to be inhibited by the direct action of A77 1726 on COX-2. More recently, A77 1726 was reported to decrease TNF-α, intercellular adhesion molecule-1 and COX-2 expression in synovial macrophages [17]. A77 1726 also inhibited IL-1β, TNF-α, nitric oxide and MMP-3 production in activated human synovial tissue cultures [18]. Thus, several studies indicate that A77 1726 inhibits the production of proinflammatory mediators by synovial fibroblasts.

Methotrexate also exhibits many of these effects, and in addition it has been shown to stimulate the synthesis of the anticatabolic factor IL-1 receptor antagonist (IL-1Ra) [19]. Increased production of IL-1Ra by joint cells in response to A77 1726 might potentially be beneficial by contributing to prevent joint damage in inflammatory arthropathies such as RA. However, it has not been determined whether A77 1726 has direct effects on the production of this anti-inflammatory molecule in synovial fibroblasts. Furthermore, potential direct effects of A77 1726 on articular cartilage and chondrocytes have not yet been examined.

In the present study we investigated the effect of A77 1726 on the production of IL-1Ra in human synovial fibroblasts, as well as in freshly isolated and in subcultured human articular chondrocytes.

In the present study we investigated the effect of the active metabolite of leflunomide – A77 1726 – on the production of IL-1Ra by human joint cells. We observed that A77 1726, while having no effect alone, markedly enhanced the secretion of IL-1Ra in the presence of IL-1β or TNF-α in synovial fibroblasts and articular chondrocytes. The effect of A77 1726 was maximal at 100 μmol/l – a dose that lies within the range of plasma concentrations that may be observed in leflunomide-treated patients [6, 23]. Because IL-1Ra has been shown to exert chondroprotective effects, our observations suggest that in the presence of proinflammatory cytokines, which are present in significant amounts in inflamed joints, A77 1726 might exert a beneficial effect by increasing the local production of this anti-inflammatory mediator by joint cells.

IL-1Ra, which was initially discovered for impeding the binding of IL-1 to lymphoma cells, is produced in four different isoforms, one secreted and three intracellular, which are derived from the same gene [24, 25]. The exact biological functions of the different IL-1Ra isoforms are still not clear [25‐27]. The major role of secreted IL-1Ra is to block the effects of IL-1 by binding competitively to IL-1 receptor I without inducing signal transduction. The intracellular isoforms may be released from cells under certain circumstances, but they have also been suggested to perform important regulatory roles within cells. Synovial fibroblasts and de-differentiated chondrocytes produce both secreted and intracellular IL-1Ra [28], and in these cells IL-1β-induced IL-1Ra production was enhanced in culture supernatants and in cell lysates in response to A77 1726. In contrast, cell lysates of freshly isolated chondrocytes contained no significant amounts of IL-1Ra, even after stimulation with IL-1β and A77 1726, which is consistent with our previous observations [28, 29].

In a recent study we observed that over-expression of either the secreted or the type I intracellular IL-1Ra isoform similarly protected mice from collagen-induced arthritis, blocking inflammation and joint damage [30]. In RA, IL-1Ra has been shown to be one of the most potent agents available to decrease the progression of joint destruction [31‐33], although its effects on inflammation and symptoms are frequently considered disappointing. It is generally considered that a 10- to 100-fold molar excess of IL-1Ra over IL-1 is required to suppress completely the biological effects of IL-1, although lower amounts of IL-1Ra can significantly inhibit IL-1-induced responses [34]. In the present study, the levels of IL-1Ra produced by synovial fibroblasts and de-differentiated chondrocytes on stimulation with A77 1726 and IL-1β usually ranged between equimolar concentrations and a twofold molar excess of IL-1Ra over IL-1. Even higher molar ratios of IL-1Ra:IL-1 were obtained when IL-1 was combined with TNF-α. Although large amounts of IL-1 are needed to obtain maximal catabolic effects in vitro, multiple lines of evidence (for example [35]) indicate that even very low levels of catabolic cytokines, including IL-1, can synergize to induce substantial effects. In vivo, it is likely that multiple cytokines present in low amounts act in synergy to induce proinflammatory and catabolic effects. Thus, the blockade of low amounts of IL-1 might be sufficient to decrease such a synergistic effect in vivo. The increased production of both secreted and intracellular IL-1Ra, which was observed in joint cells in response to A77 1726, might therefore be potentially beneficial by contributing to prevent joint damage in inflammatory arthropathies such as RA. In this regard, administration of leflunomide has been shown to limit joint destruction and improve function scores significantly, and to a greater degree than with methotrexate, according to at least two studies [1, 36]. The mechanisms that result in this protection are likely to be multiple. However, a stimulatory effect on IL-1Ra synthesis might be particularly relevant, given the important role of IL-1 in joint destruction [37].

We investigated putative pathways involved in mediating the stimulatory effect of A77 1726 on IL-1Ra production, first focusing on the known effect of A77 1726 on pyrimidine synthesis. Addition of exogenous uridine did not significantly modulate the effect of A77 1726, suggesting that it was unlikely to be related to the inhibition of pyrimidine synthesis.

A77 1726 was previously reported to inhibit COX-2 activity [11]. Also, the findings reported here confirm a previous report that 100 μmol/l A77 1726 completely blocked IL-1β-induced PGE₂ production in synovial fibroblasts [12]. Similarly, we observed that A77 1726 inhibited IL-1β-induced PGE₂ production in chondrocytes. Interestingly, IL-1β triggered production of lower amounts of PGE₂ in freshly isolated chondrocytes than in de-differentiated chondrocytes, suggesting low levels of expression and/or activity of COX-2 in these primary cells. This observation substantiates a recent report, which described a similar, differentiation stage-dependent regulation of COX-2 expression and PGE₂ production in rabbit articular chondrocytes [38]. Indomethacin increased IL-1β-induced IL-1Ra production in synovial fibroblasts and de-differentiated chondrocytes, suggesting that inhibition of COX-2 may indeed enhance IL-1Ra production in the presence of IL-1β in these cells. However, the stimulatory effect of indomethacin was repeatedly less potent than that of A77 1726. In addition, indomethacin did not affect IL-1Ra production in primary chondrocytes. This observation is consistent with the low levels of PGE₂ produced in these cells, which are suggestive of low COX-2 expression/activity. It is also in agreement with a previous study that described a lack of effect of indomethacin on IL-1Ra production in IL-1-stimulated OA chondrocytes [39]. Moreover, both in synovial fibroblasts and in chondrocytes, we observed that IL-1β-induced PGE₂ production was strongly inhibited at relatively low concentrations (10–50 μmol/l) of A77 1726, as compared with the higher doses (50–100 μmol/l) that were required to enhance IL-1Ra production efficiently (data not shown). The dose dependency of these two effects thus appeared to be slightly different. Taken together, these observations strongly suggest that, in addition to the inhibitory effect of A77 1726 on COX-2 activity and PGE₂ production, other mechanisms contribute to its stimulatory effect on IL-1Ra secretion.

Because high doses of A77 1726 have been reported to inhibit different types of receptor and nonreceptor tyrosine kinases [13‐15], we assessed whether tyrosine kinase inhibitors would affect IL-1β-induced IL-1Ra production in fibroblasts and chondrocytes. Two types of inhibitors were tested: genistein, a broad range tyrosine kinase inhibitor; and PP1, a more specific inhibitor of the src family of tyrosine kinases, the members of which have been reported to mediate IL-1 signalling in various cell types [40, 41]. Both inhibitors decreased IL-1β-induced IL-1Ra expression in synovial fibroblasts and chondrocytes (data not shown). These observations suggest that the increase in IL-1Ra production observed in the presence of A77 1726 is unlikely to be due to the inhibition of tyrosine kinases. Thus, although our findings suggest that part of the effect of A77 1726 on IL-1Ra production occurs through the inhibition of COX-2 activity, other unknown mechanisms, which remain to be characterized, are likely to be involved.

A77 1726 enhanced the production of IL-1Ra by human synovial fibroblasts and articular chondrocytes in the presence of the proinflammatory cytokines IL-1β and TNF-α. This might explain some of the protective effects exerted by leflunomide in RA and other cartilage-damaging diseases.