IFN-beta to IFN-beta switch: The only reason to switch between IFN-beta preparations is for local, or systemic, intolerance. I have many patients with local skin, or injection site, reactions who have moved to intramuscular IFN-beta-1a (Avonex) that does not have local skin reactions. In contrast, I have had many patients move from IFN-beta-1a (Avonex) to IFN-beta-1a (Rebif) or -1b (Betaferon) due to persistent flu-like reactions. Because Avonex allows the interferon receptors to regenerate before the next injections mild flu-like symptoms can persist; this does not happen with Rebif and Betaferon. However, as Rebif is given 3x per week some patients experience mild flu-like side effects after the 2-day injection break and not after the 1-day break.
NAbs: Please note that if you have developed NAbs to one IFN-beta preparation these cross-react with the other preparations. In this situation NABs would simply neutralise the effect of new IFN-beta formulation.
Lack of efficacy: I would not recommend switching between IFN-beta preparations because of lack of efficacy or perceived lack of efficacy. If you have had a suboptimal response to one IFN-beta preparation it would make sense to switch classes. In general, I tend to escalate treatment rather than switch to another moderate efficacy DMT.
Other DMTs: Provided the baseline screening bloods are fine and there are no specific contraindications I see no reason why IFN-beta can't be used after any of the other licensed DMTs. Apart from NABs inhibiting the action of other IFN-beta formulations, I am not aware that a failure to respond to any of the other DMTs predicts a lack of response to other IFN-beta. However, if you are switching due to a suboptimal response I would recommend a more efficacious IFN-beta. There is reasonable real-life data that shows switching upwards (escalation) gives a better overall response rate than switching to a similar efficacy DMT (horizontal switching).
Special circumstances: The presence of some specific comorbidities or adverse events may make it difficult to switch from one DMT to IFN-beta. For example, a persistent low lymphocyte count (<800), low neutrophil count (<1,000), low total white cell count (<1,500) and abnormal LFTs, which can occur with several DMTs, are relative contraindications to IFN-beta. Similarly, the presence of a monoclonal gammopathy is a contraindication to IFN-beta due to the risk of capillary leak syndrome.