Parse datetime with lubridate

I am trying to parse the following datetime with the following format:

library(lubridate)
a <- "2004-05-07 18:24:58.666424"

I tried the following, but returned NAs

b <- lubridate::mdy_hms(a)
c <- lubridate::mdy(a)

Could anyone please explain how to parse this. I am also fine if lubridate is not used.