Anti-TNF agents have clear therapeutic benefit in some IMIDs while they have relatively weaker or no effect in other diseases. Even in diseases where anti-TNF treatments have been shown to be successful, more than one third of patients do not respond to treatment. This lack of responsiveness may be due to a different mechanism(s) of disease in some patients. In other words, chronic joint or intestinal inflammation clinically diagnosed as RA or IBD, respectively, actually represent different pathological disease entities. Alternatively, TNFα may play a pathogenetic role only in certain stages of disease.
Another important area is to understand why and how certain anti-TNF agents have better outcomes in one disease and not in another. For instance, ETA is an important drug in the treatment of patients with RA, but a large clinical trial showed that it had no effect at the dose regimen used on decreasing the gut inflammation in IBD. In vitro studies attempted to explore differences in binding to tmTNF between ETA and INF. One must take care, however, to interpret the results of in vitro studies carefully, as they are not always generalizable to patients with complex chronic diseases. Furthermore, the results of many mechanistic studies of anti-TNF biologics vary because of the conditions under which they are run. The consequences of membrane binding depend largely on the systems in which these mechanisms are tested. Several of these studies have been done using cell lines into which a gene encoding non-cleavable mTNF has been transfected. When tmTNF is transfected into cells, investigators take care to protect it from an enzyme that would otherwise cleave the TNF molecule, called TNFα-converting enzyme (TACE) [67
]. At first the gene encoding tmTNF was transfected as a deletion mutant, but more recently, Harashima et al. [68
] developed a method of transfecting cells with a tmTNF rendered non-cleavable by site-directed mutagenesis. Using these methods, investigators assessed the number of anti-TNF molecules that can bind tmTNF either by flow cytometry or by radioassay [16
TNF exists in monomeric and biologically active trimeric forms. INF is able to bind to both monomeric as well as trimeric forms of TNF, but because of its binding site on TNF occurs in the cleft between subunits, ETA can only bind to the trimeric form [16
]. This binding configuration may explain why the number of mTNF molecules bound by INF is 1.5 to 3 times as many as ETA, while their affinities in monovalent interactions are similar. Scallon et al. [16
] performed pulse–chase experiments of radiolabeled INF and ETA and found that while both bound tmTNF, ETA had a higher off-rate, signifying that the rate of exchange of ETA with tmTNF is higher than that of INF. A recent study from the Abbott Laboratories compared the binding properties of the three FAD-approved anti-TNF agents [69
]. Using a BIAcore 3000 instrument, they found similar affinities of the three antagonists for soluble TNF, but with differing on-rates and off-rates. ETA had a faster binding on-rate and a faster dissociation off-rate to soluble TNF than INF or ADA. All three TNFα antagonists also bound to membrane TNF with similar affinities. Other factors, such as the ability of ETA to bind both TNFα and TNFβ and the ability of mAbs to bind only to TNFα, might also explain some differences in treatment outcomes using these agents.
The cross-linking of anti-TNF agent on the cell may elicit reverse cell signaling, causing reduced production of inflammatory cytokines, and increased apoptosis and growth arrest. If the surface density of mTNF on the cell is high, it would be expected that there is more opportunity for Ab cross-linking [66
]. ETA may have greater efficacy in the treatment of patients with RA because circulating rheumatoid factor as well as complement component C1q may act to cross-link this Ab when bound to lymphocytes in vivo.
Some authors argue that INF binds to both tmTNF as well as soluble TNFα, while ETA binds primarily to soluble TNF, and so cannot mediate caspase-dependent apoptosis [17
]. Others claim that both INF and ETA bind to the uncleavable form of membrane bound TNF, but INF binds with higher avidity [18
]. By binding to cells bearing membrane-bound TNF, INF is able to induce apoptosis of those cells [70
]. Monocytes isolated from the peripheral blood of patients with CD undergo apoptosis after binding of INF in vitro, i.e. a phenomenon associated with increases in caspase 3 activation [20
]. Similarly, T lymphocytes maintained in a mixed lymphocyte reaction with monocyte-derived dendritic cells show significant apoptosis in the presence of INF. Finally, binding may not predict the cellular effects. For example, a recent study found that ETA, INF, ADA, and certolizumab all bound equally to activated PBT and PBMC, but only ETA, INF, and ADA were able to induce apoptosis in those cells [14
Genetic makeup of individuals may dictate response to biological treatments. For example, RA patients with a TNFα-308G/G genotype are better infliximab responders than are patients with A/A or A/G genotypes. TNFα-308 genotyping may be a useful tool for predicting response to infliximab treatment [71
]. This observation has been extended to other anti-TNF agents independent of the treated rheumatic disease (RA, PsA or AS) [72
In summary, more than one third of patients (suffering from any approved indication) do not benefit clinically from anti-TNF treatment. Future studies should investigate whether this limitation is due to different pathogenetic mechanisms in different patients or due to involvement of different mediators of disease development and progression in different stages of disease. Other limiting factors could be the genetic make up of individuals or specific properties of the anti-TNF agent used for the treatment.