Brothers Arash and Kamiar Alaei, whose work on HIV treatment and prevention programmes earned international recognition, were arrested in June 2008 for allegedly trying to overthrow the Islamic regime.

Kamiar Alaei, 37, a Harvard University alumnus, was released in October after serving a three-year prison term but his brother Arash, 42, remains in Tehran's Evin prison to complete his six-year sentence.

The brothers fell foul of the Iranian authorities because they participated in several international HIV treatment programmes and conferences held in the US.

"We were never involved in politics. We were simply doctors trying to serve our people good, our arrests came as a surprise," Kamiar Alaei told the Guardian by phone from the US.

The Alaei brothers started their work in 1997 in a small clinic in their home town, Kermanshah, at the time when talking about HIV/Aids and using condoms were taboo in Iran.

Under the former reformist president Mohammad Khatami, the Alaeis received government help to open Aids clinics, distribute condoms and establish needle-exchange programmes nationwide.

But when Mahmoud Ahmadinejad became president in 2005, the government cracked down on individuals and institutes who were communicating with the outside world.

As a neighbour to Afghanistan, the leading producer and supplier of opium to the world, Iran is one of the largest consumers of illicit drugs, especially heroine and opium. According to Alaei, 70% of those diagnosed with Aids in Iran are drug users. In recent years, Iranian officials, however, have resorted to public execution to tackle the country's drug-related crimes.

According to Alaei, officials acknowledged that 23,000 people in Iran are living with Aids – but unofficial reports put the figures between 80,000 and 100,000.

After their arrests, the brothers were kept for several months in solitary confinement and later sentenced to prison after what Amnesty International described as a one-day "unfair trial". Inside jail, Alaei said, the two brothers worked with inmates to improve their health.

A petition in support of Arash Alaei, launched by the US-based Physicians for Human Rights (PHR), has been signed by prominent medics including two Nobel prize laureates, Françoise Barré-Sinoussi and Sir Richard J Roberts.

"Their landmark efforts to bring patients, communities and families together to reduce stigma and provide comprehensive Aids prevention and treatment are remarkable and remain singular achievements in healthcare in Iran. We only hope that Dr Arash Alaei can be freed to continue this critical and groundbreaking work," said Susannah Sirkin, PHR's deputy director.

Thanks to the work of the Alaei brothers, the UN Programme on HIV/AIDS described Iran's needle-exchange programme as one of the "clear examples of courageous, visionary leadership in the response to HIV" in its 2008 Report on the global AIDS epidemic.