'Rac'-ing upstream to treat rheumatoid arthritis

Signal transduction pathways regulate the production and function of many cytokines implicated in immune-mediated diseases. Targeting these enzymes with small molecule inhibitors represents a fertile field for the treatment of rheumatoid arthritis. Recent successes with compounds that block upstream kinases suggest that proximal members of the signaling cascades, such as Rac and other Rho family enzymes, might have therapeutic potential. Balancing efficacy and toxicity, however, remains a significant challenge that will require careful evaluation.

Protein-based therapeutics for rheumatoid arthritis have limitations despite improved clinical outcomes. In addition to expense and the need for parenteral administration, a signifi cant percentage of patients do not have robust responses. Intracellular signaling molecules, such as members of the Rho family [1], represent an attractive alternative because the compounds are often orally bioavailable and can block numerous proinfl ammatory mediators simultaneously.
Targeting signal transduction, however, has been an exercise in frustration until recently. Th e p38 mitogenactivated protein kinase saga is emblematic of these problems [2]. Despite abundant preclinical data supporting the utility of p38 inhibitors, benefi t has been marginal at best [3]. It is important to recognize that success in biologics also did not come with the fi rst attempt. Numerous failures preceded the advent of TNF blockers, including anti-CD4, anti-CD5 and anti-CD52 antibodies, IL-2-diphtheria toxin fusion protein, IFNγ, IL-2, and several others. Clinical effi cacy for JAK and Syk inhibitors demonstrated in recent years crossed the Rubicon for signaling-directed therapeutics [4,5]. Th e question now is not whether some of these agents can be eff ective; rather, it is whether the toxicity and side eff ects will be acceptable in a world where biologics have an advantageous therapeutic index.
A distinguishing feature of the encouraging interventions (Syk, JAK, and perhaps c-Kit) compared with p38 inhibitors is that the former targets are proximal in the signaling cascade. Going upstream can be risky, since each enzyme casts a broader penumbra of eff ects than a downstream target. Th is increases the potential for both benefi t and toxicity. Risk, however, can be managed; lack of effi cacy cannot.
Th is lesson is being exploited by going far upstream using therapeutics that inhibit the Rac proteins. Th ese signaling enzymes, unlike the classical protein kinases that phosphorylate various transcription factors, are GTPases in the Rho family [6]. Th ey regulate a vast array of functions, including cell movement, proliferation, adhesion, and phagocytosis. Many of these functions result from the subsequent activation of downstream protein kinases, such as the mitogen-activated protein kinase family. Blocking Rac proteins, such as Rac1, could potentially suppress many mechanisms implicated in rheumatoid arthritis.
Tak and colleagues approached this problem with a peptide inhibitor in order to explore in vitro and in vivo eff ects of Rac1 inhibition [1]. Th e peptide decreased production of key cytokines like IFNγ, TNF, and IL-17 by cultured T cells. Th ey also examined the peptide's eff ect in collagen-induced arthritis, a standard mouse model of rheumatoid arthritis [7].
Th e use of peptide therapeutics in vivo is fraught with problems, such as a short half-life (often only minutes) and limited access to the intracellular space where the target actually resides. Despite this limitation, a modest decrease in paw swelling was observed along with a lower anti-type II collagen antibody titer. Interestingly, no signifi cant eff ect was observed on the clinical arthritis scores or histologic evidence of joint infl ammation and damage. If therapy was delayed until after disease was established, a nonsignifi cant trend toward decreased paw swelling was noted.
Several aspects of the study warrant comment. Th e lack of eff ect on clinical scores is interesting, as this usually

Abstract
Signal transduction pathways regulate the production and function of many cytokines implicated in immunemediated diseases. Targeting these enzymes with small molecule inhibitors represents a fertile fi eld for the treatment of rheumatoid arthritis. Recent successes with compounds that block upstream kinases suggest that proximal members of the signaling cascades, such as Rac and other Rho family enzymes, might have therapeutic potential. Balancing effi cacy and toxicity, however, remains a signifi cant challenge that will require careful evaluation.
tracks with paw swelling. Th ese two endpoints, however, evaluate somewhat distinct phenomena. Th e former measures edema or tissue hyperplasia in a single joint (usually the ankle), while the latter determines the sum of the total number of active joints. It is possible to have relatively mild arthritis (and minimal swelling) with a high clinical score. Conversely, severe disease in the ankles but nowhere else could also lead to disparate outcomes. Th e two indices of disease can thus provide complementary information. In this case, the lack of eff ect on joint destruction and synovial histology suggests that the Rac1 inhibitory peptide might be acting through vascular leakage and tissue edema rather than immune cell infi ltration into the joint.
A second important point is that animal models are an imperfect representation of rheumatoid arthritis. Th e kinetics of the synovial signaling pathway in mice is compressed compared with human disease, and the specifi c kinases engaged can vary from model to model [8]. Animal data must therefore be interpreted with some caution. Nevertheless, results for the Rac1 inhibitory peptide off er a signal of effi cacy even though they probably underestimate the potential benefi t. A therapeutic agent with a longer blood half-life that is also optimized for cell penetration could give substantially better results. Th e safety of blocking Rac1 cannot be accurately gauged with the peptide for the same reasons.
Overall, the future looks brighter for blocking signal molecules than it did a few years ago. With the plethora of potential targets, such as Rac, and armed with information on the biology of upstream rather than downstream molecules, there is renewed optimism for developing new therapeutics for rheumatoid arthritis.