The invention relates to a method for determining whether a compound is a neurotrypsin inhibitor, characterized in that the compound is incubated together with neurotrypsin, a variant thereof or a fragment comprising the protease domain and with a protein or peptide comprising agrin, a variant thereof or a fragment comprising the α- or the β-cleavage site of agrin, in an aqueous buffer solution, and the amount of cleavage of agrin is measured. Additionally, the invention relates to inhibitors of neurotrypsin found by this method, in particular to compounds of formula wherein Hal1 and Hal2 are fluorine, chlorine or bromine, and the use of such inhibitors for the treatment and/or prophylaxis of diseases caused by deficiency of synapses, for example skeletal muscle atrophy, schizophrenia, and cognitive disturbance.