Nektar Therapeutics (NKTR - Get Report)  shares dropped on Tuesday after Goldman Sachs double-downgraded the biopharma to sell from buy and slashed its one-year price target by nearly two-thirds on expectations its in-the-works cancer treatment won't generate revenue anytime soon.

Nektar slumped 13% to $16.18 after Goldman analyst Paul Choi downgraded the stock and slashed his price target to $16 from $54.

The analyst sees "execution missteps outweighing announcements of mostly positive clinical developments." 

Among those missteps were manufacturing issues related to its bempegaldesleukin cancer drug as well as reduced confidence in the company's ability to execute on the drug's late-stage development.

$NKTR will be at the #CICON2019 conference this week presenting data on our #immunotherapy bempeg and nivolumab in metastatic, triple-negative #breastcancer. Find details here: https://t.co/JTPnWMl7hQ #cancerresearch #oncology

— Nektar Therapeutics (@NektarNews) September 24, 2019

"Nektar now faces significant challenges in rebuilding confidence in the clinical and commercial outlook for" its experimental cancer drug bempegaldesleukin combined with Bristol-Myers Squibb's  (BMY - Get Report) Opdivo, Choi wrote in a note to clients.

Shares of Nektar have lost about half their value since December, compared with an 11% rise for the S&P 500.

The stock has shed more than $14 billion from a March 2018 peak. The company also lost its standing in the S&P 500 last month.

Separately, Choi also downgraded Puma Biotechnology (PBYI - Get Report) to sell from neutral and slashed his price target to $8 from $24 amid what he sees as potential competition for the company's Nerlynx cancer drug.

That stock has lost nearly 80% in the past 12 months.