Unlike in RA, combination with MTX has not been demonstrated in clinical trials (eg IMPACT, ADEPT) to be superior to anti-TNF monotherapy, simply because MTX was not mandated in the placebo arm. This in turn is because MTX ...has not been shown to be effective in treating PsA peripheral arthritis in the MIPA trial. But this trial used what is today considered to be homeopathic doses of MTX (15mg/wk), especially considering that we may choose to hike anti-TNF doses to achieve better induction in the more severe PsA.
So, the task of elucidating whether combination with MTX is better (and therefore necessary, as in RA), is left to post-marketing forward observational studies and registry-based multiple regression analysis. These studies are subject to much selection bias and questionable statistical acrobatics, but this is a topic for another day's post.
As such, the conclusion of this study that combination with MTX in unnecessary should await confirmation (or refutation) in future prospective randomised double-blind placebo-controlled trials.
Till then, I can think of at least 3 reasons why we should combine anti-TNF treatment in PsA with a traditional DMARD like MTX:
1) It may help to prevent the development of neutralising antibodies to the anti-TNF;
2) It may allow for eventual maintenance on MTX alone, doing away with the risks, high cost and inconvenience of a biologic;
3) In clinical practice (mine at least), it is synergistic with the biologic in achieving better disease control especially in the more severe cases, and especially if given parenterally at high doses (eg subcutaneous MTX 25mg/wk).
2015 saw a slew of new targets and options in the treatment of Psoriatic Arthritis and Psoriasis. The Th17 pathway is the game changer.