Two women prison doctors among five killed in Afghan bombing
The blast happened in the south of the capital as they were on their way to Pul-e-Charkhi prison, Kabul police spokesman Ferdaws Faramarz said.
Faramarz had earlier told reporters that four prison doctors had been killed by the bomb.
Hundreds of Taliban fighters and other criminals are incarcerated at Pul-e-Charkhi, on the eastern outskirts of Kabul.
President Ashraf Ghani condemned the attack.
"The terrorist groups including the Taliban who have faced defeat on the battlefields are targeting civilians in the cities, highways and public facilities," he said in a statement.
"This is a sign of their desperation and defeat."
No group claimed the attack but the Taliban denied involvement.
Kabul has been hit by a wave of deadly violence in recent months despite the Taliban and the government engaging in peace talks.
Targeted killings of prominent figures, including journalists, politicians and rights activists have also become more common recently in Kabul and other provinces.
The jihadist Islamic State group has claimed responsibility for some of the recent attacks in the capital, which have killed dozens of people.
Tuesday's attack came just two days after a car bomb targeting a lawmaker killed 10 people and wounded more than 50 in Kabul.
Lawmaker Khan Mohammad Wardak was wounded in that blast.
Days before that incident, deputy Kabul province governor Mahbobullah Mohebi was killed in a similar attack.
On Monday, an Afghan journalist was shot dead by gunmen in the eastern city of Ghazni in what appeared to be another targeted killing.
Rahmatullah Nekzad had contributed to the Associated Press news agency since 2007 and also previously worked with the Al-Jazeera network.
The same day, Afghan officials said security forces beat back a Taliban assault further north in Faryab province.
The insurgents detonated a car bomb that killed three security personnel in Dawlat Abad district, according to provincial police spokesman Karim Yourish.
Officials in northern Kunduz province also reported sporadic clashes after two separate checkpoints were attacked by the Taliban, with authorities providing differing figures on casualties.