It's been a long time coming but Roche (OTC:RHHBY) and ImmunoGen's (NASDAQ:IMGN) breast cancer drug T-DM1 is finally approved for sale in the U.S., with a new name, Kadcyla, to boot.

You'll recall -- or maybe you won't if you haven't been investing in the biotech space that long -- that the Food and Drug Administration refused to even look at the application for approval back in 2010. Roche tried to gain accelerated approval based on phase 2 data, but the FDA concluded that the company hadn't tested the drug on patients that had exhausted all their options.

Roche ran a phase 3 trial, which Kadcyla passed with flying colors. The trial would have been necessary even if the FDA had issued an accelerated approval, but Roche could have been collecting revenue from the drug for two years while it waited for the data. ImmunoGen could have collected the $10.5 million milestone payment tied to the approval and started collecting royalties on the sales.

ImmunoGen contributed the DM1 part of T-DM1, a toxic payload conjugated to Roche's antibody using the biotech's Targeted Antibody Payload technology. Kadcyla isn't the first antibody-drug conjugate to hit the market; Seattle Genetics' (NASDAQ:SGEN) Adcetris was approved in 2011. The main difference is that Adcetris is based on a previously unapproved antibody, while Kadcyla is a toxic payload linked to Herceptin, which racked up $1.8 billion in U.S. sales last year. Adcetris managed less than a tenth of that albeit treating a much smaller market.

Kadcyla has better efficacy and fewer side effects than Herceptin, but it isn't clear how much of the multibillion-dollar market Kadcyla will take right away. Roche recently got another breast cancer drug, Perjeta, approved for use in combination with Herceptin. ImmunoGen might have to wait until data from a trial testing Kadcyla with Perjeta reads out before Kadcyla starts producing big-time royalties for the biotech.

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