A special case of fragment linking is dimerization, in which two copies of the same fragment bind to adjacent sites in a protein and are subsequently linked together (see for example here, here, and here). A recent example was published in J. Med. Chem. by Bernard Pirotte, Julien Hanson (University of Liège), Lionel Pochet (University of Namur), Jette Kastrup (University of Copenhagen) and their collaborators.
The researchers have for some time been interested in AMPA receptors, critical components in neuronal synaptic transmission. Increasing their activity could be useful for treating diseases such as depression and schizophrenia, but increasing activity indiscriminately is known to be toxic. One approach has been to develop positive allosteric modulators (PAMs), which increase the activity only in the presence of the natural ligand glutamic acid, thus amplifying the normal biological signal.
AMPA receptors themselves are dimers of dimers. Many different PAMs have been reported for AMPA receptors, and some of these are in fact dimeric molecules that span two adjacent binding sites across the dimer interface. A crystal structure of a molecule closely related to compound 35 revealed that each molecule binds to two adjacent protein subunits, so the researchers designed compound 22, which pairs the molecules through a simple ethylene moiety. The strategy paid off with a low nanomolar activator, which crystallography confirmed binds as expected.
Interestingly, conceptually cleaving the bond connecting the two fragments generates a compound (33) which is slightly less active than the initial fragment 35; it is possible the methyl groups are too close to one another when two copies of compound 33 are bound.
As the researchers point out, compound 22 is one of the most potent AMPA receptor PAMs reported. However, it is also quite large, particularly since it needs to cross the blood-brain barrier. No animal data are reported, but a simple metric called the CNS MPO desirability score is reasonably predictive. This score is based on the molecular weight, lipophilicity, total polar surface area, number of hydrogen bond donors, and basicity; higher scores are better. By this measure, compound 22 is predicted not to have high brain penetration, though of course any metric needs to be taken with caution.
However, a separate J. Med. Chem. paper by many of the same researchers revealed that dimerizing the molecules is not essential: simply growing compound 35 could also generate a low nanomolar AMPA receptor PAM (compound 8). Crystallography revealed that the added phenyl group binds where the second molecule of compound 35 would normally bind. Moreover, compound 8 has a higher ligand efficiency as well as a higher CNS MPO desirability score than the dimeric compound 22, suggesting that it is more likely to be able to cross the blood-brain barrier.
In the absence of pharmacological or pharmacokinetic data, if forced to choose I would probably focus on compound 8 rather than compound 22. All of which is to say that although there is a certain elegance to dimerizing molecules, you might be able to replace one of them with a smaller, simpler moiety.