Through its US subsidiary, Astellas will pay $387m (€261m) upfront and a maximum of $150m later, depending on various milestones.
However, that will feel like small change if its plan to become one of the world's leading antibody drug research houses comes to fruition.
Using data from IMS Health, Astellas has predicted that sales of cancer drugs will double between 2005 and 2015 to over €21bn and believes the majority of this growth will be down to antibodies and molecular targeted drugs.
The two sectors are predicted to account for over €6bn of that growth (Y420bn (€2.6bn) and Y129bn to Y913bn and Y615bn for antibodies and targeted therapies respectively).
"Agensys will be the cornerstone of our biologics efforts and an integral component of building our oncology efforts within our franchise," explained Masafumi Nogimori, CEO of Astellas.
Not only that, but Astellas scientists can also use the targets identified by Agensys in their small molecule drug discovery work.
That amounts to some 30 different targets across 14 different types of cancer.
A further bonus in the deal is that because Agensys already had one antibody in the clinic, the firm also has a GMP manufacturing facility, which can produce enough drug to complete Phase I and early Phase II clinical trials.
The facility includes cell culture and purification instruments.
Agensys' most advanced drug candidate is AGS-PSCA, which is a fully human monoclonal antibody that targets Prostate Stem Cell Antigen (PSCA), which is found on tumour cells from the majority of patients with all stages of prostate, pancreatic and bladder cancers.
The company also has two more antibodies in Phase 0 and eight in preclinical testing.
The antibodies themselves are generated using the firm's XenoMouse technology, which was in-licensed from Abgenix.
One of the inventors of the system, Dr Jakobovits, joined Agensys back in 1999 as chief scientific officer.
The firm has also licensed Seattle Genetics' linker-toxin technology in order to be able to generate antibody-drug conjugates.
To identify a useful target, the team at Agensys use a bank of both patient-derived and healthy tissues.
Using these, instead of established cell lines, helps the firm find novel targets it claims.
The levels of gene activity are assessed at different stages of tumour development to find potential new targets.
These are then validated using immunohistochemical staining in tissue arrays, and other analysis techniques including small interfering RNA (siRNA).
As part of the strategy to up their antibody work, Astellas has already splashed its cash twice this year.
Back in March, it bought a non-exclusive license to Regeneron's VelocImmune technology and, in the same month, bought access to a phage display library from MorphoSys.