Merging Fragments for Matriptase

We often talk about methods here: how to screen, how to prosecute those actives, and everything in between.  This is one of those what you do with the actives posts.  In this paper, a group from Aurigene and Orion present their results on Matriptase.  There have been multiple reported matriptase inhibitors, small molecule and peptide based.  Previous work from this group showed compounds that were active in cell-based migration and invasion assays and in mice with tri-substituted pyridyls and benzene compounds.  For this work, they take a SBDD approach:  "structure divulges a trypsin-like S1 cavity, a small hydrophobic S2 subpocket, and a solvent exposed spacious S4 region."

In screening benzamidine fragments (MW less than 300) they found 2 actives, ~80uM (Figure 1).

Figure 1.  Benzamidine screening actives

These were modeled in to the active site and obviously the amidine moiety went into S1.  Cpd 1's benzene moiety went nicely into S4 while 2's piperidyl went into S1'.  S4 easily accepted the more hydrophobic napthyl instead of phenyl and then they decided to see if the napthyl compound and 2 could be "linked" and the beta carbon. [So, my first quibble here is that this is not really a linking approach; this is fragment merging. Linking involves modeling, SBDD, and discovery of different linkers.  Its very difficult to do without specialized methods.  What they did here was see huge spatial overlap of compounds and voila, "we can add something right here".]  Well, not surprisingly, this worked.  They describe their SAR around each pocket to pick the compounds to merge, go read it if that interests you. The did crystallize the merged compound and it confirmed the modeling.  The final compound showed activity in cell-based assays and in mice.  That's good.  

This work can be summarized as follows: if you have significant spatial overlap you have a very good chance of merging disparate moieties.  So, two things bother me here.  First, the actives 1 and 2 are mighty big for fragments (more than 22 HAC).  That's fine, tomato...to-mah-to.  The final compound ends up pretty honking big too (37 HAC).  What is really bothersome, at least to me, is the LE.  Both actives start well below 0.2 (for a protease!) and they never improve on it. Now, Pete may disagree, but metrics have a place in FBDD.  Does the LE metric in this case tell us anything?