Ciprofloxacin and metronidazole exert an antibacterial activity against Enterococcus faecalis (E.f.). However,
applying paste containing these antibiotics into the root canals is not convenient for endodontists. This study aimed
to study the possibility of hydroxypropyl methylcellulose (HPMC) as a vehicle for antibiotic delivery. The minimum
inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the two antibiotics were determined.
A 1:1 ratio of each antibiotic was mixed with 2 % HPMC to form a gel, and the in vitro drug release was tested by
a Franz diffusion cell. Fifty-four roots of the mandibular premolars were mechanically instrumented by Protaper
Next rotary files until X3 and divided into three groups (18 each) (i) non-infected roots, treated with gel base, (ii)
infected roots with E.f. for 21 days, treated with gel base, and (iii) infected roots with E.f. for 21 days, treated with
gel containing the combined antibiotics for 14 or 28 days. DNA and RNA were isolated from the ground roots. The
absolute quantity of E.f. DNA and relative mRNA expressions of E.f.-specific sequence and pbp5 were determined by
qPCR and RT-qPCR, respectively. MIC and MBC of the double antibiotic solution were 5 and 250 μg/ml, respectively.
There was a significant decrease in the E.f. DNA content in group (iii) at 14 and 28 days (p<0.001). mRNA expression
of E.f.-specific sequence was significantly reduced in group (iii) at both periods (p<0.01), whereas pbp5 expression
was significantly increased (p<0.01). This study demonstrated an in vitro efficacy of the combined antibiotics in the
HPMC gel against E.f., proposing an application for endodontic treatment.