In this program, Doctor Lucas and Doctor Phlox essentially traded places; Lucas was assigned to a city on Denobula while Phlox went to Starfleet Command on Earth. The two maintained a long-distance friendship through the occasional subspace letter.
In September 2151, Lucas mailed Phlox to tell him of his stress during the violent Denobulan mating season. Phlox reciprocated with a story about his new posting on Enterprise NX-01, specifically the outbreak of a plague on Valakis. (ENT: "Dear Doctor")
After the Xindi attacked Earth in March 2153, Doctor Lucas was recalled to his home planet to help deal with the devastation. Though he sent a letter to Phlox telling him of this, Phlox was unable to respond because of Enterprise's busy involvement in a search for the Xindi weapon. Phlox finally responded in January 2154, as he caught up on his correspondence while the rest of the crew spent four days in comas. (ENT: "Doctor's Orders")
Later that year, Doctor Lucas was assigned as senior medical director of Cold Station 12, a Medical Exchange facility that housed dangerous pathogens in a secure environment. In May 2154, the facility was attacked by Doctor Arik Soong and his Augments, seeking access to Augment embryos on the station. Lucas refused to give Soong the access codes, even when Soong killed one of the C-12 staff with Symbalene blood burn. Lucas remained defiant until Malik threatened to kill Phlox, who had arrived on the station as part of a rescue team. (ENT: "Cold Station 12")
In the writers' first draft script of "Dear Doctor", Lucas' involvement in the Interspecies Medical Exchange was referred to only in the past tense, with the script establishing he had met Phlox through the IME. Rather than serving on Denobula as part of the exchange program, he was referred to as currently doing a fellowship at the research hospital where Phlox had done his training. The same draft of the script didn't have Lucas speak voice-overs intermittently throughout the episode, as in the final version of the installment, as Phlox instead did so.
In the final draft script of "Cold Station 12", Lucas was described as being in his fifties as of 2154.