Due to their marked larvicidal activity, macrocyclic
lactones (MLs) are used for the prevention of
heartworm disease ( Dirofilaria immitis) in dogs. They have also been shown to eliminate adult parasites after long-term administration, with a so-called "slow-kill" effect. In addition, recent studies have established that a combination of
doxycycline, which eliminates the endosymbiont Wolbachia, and MLs has superior adulticide effects when compared to MLs alone. It has been hypothesized that the apparent synergism between
doxycycline/MLs may be due to interaction with drug efflux
transport proteins. The aim of the present study was to evaluate gene expression of several
transport proteins in D. immitis adults treated in vitro either with
doxycycline alone,
ivermectin alone,
moxidectin alone, or a combination of
ivermectin or
moxidectin with
doxycycline for 12 h. Quantitative PCR analysis showed a sex-dependent response to treatments. In female worms, Dim-pgp-10, Dim-haf-1 and Dim-haf-5 were upregulated compared to controls with
doxycycline alone and when combined with
ivermectin.
Moxidectin did not induce any changes in gene expression. In males,
moxidectin administered alone induced a slight increase in Dim-pgp-10, Dim-pgp-11and Di-avr-14, while
ivermectin in combination with
doxycycline produced significant upregulation of the ML receptor Di-avr-14. These results suggest possible synergism between the two drug classes and different susceptibility of males vs. females to adulticide effects.