NEW YORK (TheStreet) -- August was a horrible month for biotech stocks.

Excluding penny stocks, no biotech company fared worse than NantKwest (NK - Get Report), the cancer immunotherapy vanity project fronted by health care billionaire Patrick Soon-Shiong. The market's been a nasty place for Nantworks since the record-setting IPO at $25 per share in late July. The stock closed Monday at $17.12 -- a 44% loss for the month of August, according to FinViz.com.

Sorento Therapeutics (SRNE), another biotech stock in which Soon-Shiong owns a large stake, fell 39% during August.

Biotech's other billionaires didn't fare much better during the August doldrums, notes health care investor and biotech-Twitter (TWTR - Get Report) commentator Brad Loncar. R.J. Kirk's Intrexon (XON - Get Report) and Ziopharm (ZIOP) fell 32% and 35% during the month, respectively. Phil Frost's Opko Health (OPK - Get Report) shares dropped 42%. MannKind (MNKD - Get Report), the eponymous biotech of Al Mann, escaped August rather easily, losing just 12%.

All these stocks underperformed the benchmark iShares Nasdaq Biotechnology ETF  (IBB - Get Report), which lost 11% during August.

The breadth of the selling during the month was impressive. Among all U.S.-listed biotech stocks regardless of market cap, only 68 companies managed to end August with higher stock prices, while 267 biotech stocks lost ground, according to FinViz.com.

Other notable losing stocks during August: Celldex Therapeutics (CLDX - Get Report) down 35%, OvaSciences (OVAS)  down 29%, Intercept Pharmaceuticals (ICPT) down 28% and Spark Therapeutics (ONCE - Get Report)  down 27%. The once-hot CAR-T cancer stocks Kite Pharma (KITE) and Juno Therapeutics (JUNO) lost 27% and 26%, respectively, in August. Gene therapy darling Bluebird Bio (BLUE - Get Report) fell 20%. 

Of course, none of this matters today, Sept. 1, the unofficial start of the rest of the year. It's good to note the dreadful sell-off in biotech stocks for the historical record, but it doesn't help investors pick winning stocks (or avoid further losing stocks) for the remainder of 2015 and beyond.

If you want to be an optimist, note the IBB ETF is still outperforming the broader market for the year.

IBB Chart

Happy biotech investing!

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.