From its lowest point in March 2009, Human Genome Sciences (Nasdaq: HGSI) has risen an astronomical 6,300%.

What does that tell us about biotech investing? It can be extremely profitable, but it's also extremely difficult. Human Genome was trading for less than $0.50 per share back then because few people gave its lupus drug, Benlysta, any chance at producing positive clinical trial results, let alone getting past the Food and Drug Administration.

But it did -- on both accounts. Human Genome and marketing partner GlaxoSmithKline (NYSE: GSK) announced last night that the drug had gained regulatory approval.

With the regulatory hurdle out of the way, is Human Genome a good buy? See above: Biotech investing is extremely difficult.

At a market cap of $5.5 billion, Human Genome is undervalued if Benylsta hits annual sales of $3.6 billion to $7 billion as some analysts are calling for. But there are still a couple of unknowns that will affect how well the drug will sell.

Benylsta isn't recommended for patients with lupus that has caused severe kidney or central nervous system problems. The waxing and waning nature of lupus means that some of those patients will wane enough to be appropriate for Benylsta, but whether doctors will prescribe it at that point remains to be seen.

The FDA didn't explicitly say that doctors shouldn't use Benylsta on African-Americans, which make up 25% of the patients with lupus, but did highlight the fact that the drug didn't seem to work in that population. The FDA is requiring the companies to run a placebo controlled trial in African-Americans. The results from the trial are years away but will determine the long-term sales of Benlysta.

Up-and-coming lupus drugs also have the potential to affect sales of Benlysta. While Benlysta passed its primary endpoints in the clinical trials, the efficacy isn't anything to get overly excited about. When the option is Benylsta or nothing, it'll surely sell well, but the drug has plenty of competitors following behind. UCB and Immunomedics' (Nasdaq: IMMU) epratuzumab is in phase 3 trials as is Eli Lilly's (NYSE: LLY) LY2127399. Farther back ImmuPharma and Cephalon's (Nasdaq: CEPH) Lupuzor and AstraZeneca's (NYSE: AZN) sifalimumab are both in phase 2 development.

I think it's safe to say that Human Genome won't be another 63-bagger in my lifetime, but there's still a couple of doubles left in it if it can get some help from doctors and some slip ups from potential competitors.

Interested in keeping track of Human Genome as it launches Benlysta? Click here to add it to My Watchlist, which will help you keep track of all our Foolish analysis on Human Genome.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.