Rasagiline derivatives combined with histamine H3 receptor properties.

The irreversible monoamine oxidase B (MAO B) inhibitor rasagiline has been described with multiple disease modifying effects in vitro on models of Parkinson's disease. The combination of this established drug to recently developed histamine H3 receptor (H3R) antagonist elements gives new impetus to the design of multitargeting ligands. Surprisingly, the 5-substituted 3-piperidinopropyloxy rasagiline derivative 1 was more potent on both targets than its 6-substituted isomer. It showed nanomolar affinities at the desired targets (MAO B IC50 = 256 nM; hH3R Ki = 2.6 nM) with a high preference over monoamine oxidase A (MAO A) and negligible affinity at histamine H1, H4, dopamine D2, D3 receptors or acetyl-/butyrylcholinesterases.

Ciproxifan, a histamine H3 receptor antagonist, reversibly inhibits monoamine oxidase A and B.

Ciproxifan is a well-investigated histamine H3 receptor (H3R) inverse agonist/antagonist, showing an exclusively high species-specific affinity at rodent compared to human H3R. It is well studied as reference compound for H3R in rodent models for neurological diseases connected with neurotransmitter dysregulation, e.g. attention deficit hyperactivity disorder or Alzheimer's disease. In a screening for potential monoamine oxidase A and B inhibition ciproxifan showed efficacy on both enzyme isoforms. Further characterization of ciproxifan revealed IC50 values in a micromolar concentration range for human and rat monoamine oxidases with slight preference for monoamine oxidase B in both species. The inhibition by ciproxifan was reversible for both human isoforms. Regarding inhibitory potency of ciproxifan on rat brain MAO, these findings should be considered, when using high doses in rat models for neurological diseases. As the H3R and monoamine oxidases are all capable of affecting neurotransmitter modulation in brain, we consider dual targeting ligands as interesting approach for treatment of neurological disorders. Since ciproxifan shows only moderate activity at human targets, further investigations in animals are not of primary interest. On the other hand, it may serve as starting point for the development of dual targeting ligands.