Comment on: Antagonistic in vitro interaction between olorofim and voriconazole against Aspergillus fumigatus

Dear Editor: We read with interest the report by Laloum et al. regarding their data showing an antagonistic interaction between olorofim and voriconazole against Aspergillus fumigatus in vitro.1 We would like to extend their report by connecting it with other data that were in press at JAC the time of review of their paper. We concur that in vitro antagonism can be shown between olorofim and voriconazole against many, but not all, isolates of A. fumigatus. We find their agar-based demonstrations showing that the interaction is focused within a narrow band around the MIC of the azole to be especially instructive. As called for by Laloum et al., we have extended their observations on voriconazole by publishing data examining the in vitro combination of olorofim with posaconazole, isavuconazole, fluconazole as well as amphotericin B, terbinafine and caspofungin against A. fumigatus and non-fumigatus species.2 For all isolates of A. fumigatus, A. flavus, A. terreus and A. niger we observed unidirectional antagonism of olorofim activity by isavuconazole and posaconazole whereas fluconazole had no interaction with olorofim. Similar to Laloum et al., we found that voriconazole antagonized olorofim activity for some, but not all of the isolates tested (62.5%2 and 48%1 of isolates). Similarly, van Rhijn et al.3 also observed different interactions between olorofim and voriconazole dependent on the Aspergillus isolate tested (antagonism for a wild-type isolate and indifference for an azole-resistant isolate) that was mistakenly reported as uniform antagonism in the original publication and then quoted as ‘strong antagonism’ for both isolates by Laloum et al. Taken together, there is now evidence that mould-active azoles antagonize olorofim in vitro against Aspergillus spp. but this phenomenon is not consistent across isolates/species for voriconazole and varies further across analytical methods. Further highlighting the method-, genus-, species- and isolate-dependent nature of these findings, indifference has been observed against Madurella mycetomatis4 and Scedosporium aurantiacum.5 Regarding the nature of the antagonistic interaction between azoles and olorofim, we note that Laloum et al.1 also saw that the effect was seen at concentrations of the azole at or just below the azole MIC,2 probably as a result of this condition causing upregulation of the pyrimidine biosynthetic pathway.3 However, as with most of our experiments,2 resulting raised olorofim MICs remained within the wild-type distribution for olorofim-susceptible Aspergillus spp.1,6 Some of these in vitro findings have been validated in vivo, addressing one of Laloum and colleagues’ call to action. Using a murine model of invasive pulmonary aspergillosis, Darlow et al. reported that antagonism could be shown between A. fumigatus and the mould-active azole posaconazole.7 These authors conclude that a combination of olorofim and a mould-active azole is probably efficacious in wild-type infections but may be suboptimal in azole-resistant infections where, although there would be minimal contribution of the azole to antifungal activity, it could antagonize olorofim to produce a submaximal effect. Importantly, preclinical antagonism has been noted between currently approved antifungal agents but has not precluded combination therapy.8 Laloum et al. called for the investigation of the efficacy of combination treatment in clinical settings and we have done this by examining data from our recently completed open-label, single-arm, Phase 2b study (NCT03583164, Study 32) of patients with few or no treatment options for proven IFD or probable invasive pulmonary aspergillosis based on the 2008 European Organization for Research and Treatment of Cancer/Mycoses Study Group (EORTC/MSG) definitions.9 We previously analysed this for the first 100 enrolled patients and did not find evidence of a clinical consequence in those who received an azole in combination with olorofim.10 The data from the full 203-patient dataset have now been analysed and reveal no consistent differences in outcome or safety risk when olorofim is used as monotherapy compared with its use in combination therapy with either a mould-active azole or with fluconazole; a full manuscript has been submitted to JAC alongside this letter.11 In conclusion, we thank Laloum and colleagues for confirming our preclinical findings with Aspergillus and for providing instructive visual demonstrations of the concentration-dependent unidirectional antagonism of olorofim by the mould-active azoles. Importantly, antagonism is not seen with fluconazole, amphotericin B, terbinafine, or caspofungin. Finally, no evidence of clinical consequence of these findings has been seen in analyses of data available to date. Funding for this research was provided by F2G, Limited (Alderley Park, Macclesfield, UK). The authors are employees and shareholders of F2G, Limited.