Bernie Sanders and Alexandria Ocasio-Cortez are calling on the Trump administration to block a drug company in its efforts to extend its patent on a crucial anti-HIV drug.

In a letter to the US Patent and Trademark Office shared exclusively with the Guardian, the Vermont senator and the New York representative accuse the pharmaceutical company, Gilead, of “deceitful and immoral” behavior in holding back the drug, Descovy, from the market until its patent term for an earlier, allegedly less safe anti-HIV drug had been exhausted.

“It is an absolute disgrace that in America, a greedy drug company like Gilead can deprive hundreds of thousands of Americans of lifesaving HIV medicine to extract more profit, lie about it, and then have the audacity to ask the US government to award it with a longer monopoly to reap tens of billions more in profits,” Sanders said.

“We have got to block this obscene giveaway for corporate wrongdoing. The Trump administration must not reward Gilead for this immoral behavior.”

The involvement of Sanders and Ocasio-Cortez casts a powerful spotlight on efforts by a grassroots activist group, PrEP4All Collaboration, to block a patent extension for Descovy.

The group filed a petition with the patent office last week, laying out the allegations against Gilead.

Gilead did not immediately reply to a request for comment.

In a reply last week to the PrEP4All petition, Gilead spokesman Ryan McKeel said: “Patent safety is of foremost importance to us, and any implication that Gilead delayed the development of a drug known to be safer than [the older drug] is false.”

But the letter by Sanders and Ocasio-Cortez accuses Gilead of profiteering.

“In the process of applying for patent extension,” they write, “Gilead withheld information on its true motives” for halting Descovy’s development.

“Gilead should absolutely be denied a patent extension request for failing to disclose material information to the USPTO [patent office],” Ocasio-Cortez said.

“They kept a safer drug off the market to extract profit. In doing so, they have inhibited efforts to end the HIV epidemic. I am proud to join Senator Sanders today in calling on the USPTO to deny their patent extension request.”

Gilead has made $36bn off an earlier anti-HIV drug, Truvada, since 2004, the members of Congress wrote in a letter dated 6 December.

“Corporate misconduct must not be rewarded by the US government through extending a government-granted monopoly on this medicine.”

The letter also observed that “thousands of people are unnecessarily dying from a preventable disease”, with the latest generation of anti-HIV drugs shown to be 99% effective at curbing transmission.

“If manufactured generically” Sanders and Ocasio-Cortez wrote, “Descovy could quickly and affordably reach hundreds of thousands of Americans.”

If the government extends Gilead’s patent, the corporation could use its monopoly to charge several thousand times more for the same compound, the members of Congress said.

That, they added, would put it out of reach for untold numbers of Americans desperate for affordable HIV treatment and prevention.