Transthyretin Leu55Pro is one of the most aggressive mutations in familial amyloidotic polyneuropathy, an autosomal dominant disorder characterized by extracellular deposition of fibrillar amyloid protein. This variant has the ability to form fibrils in vitro under physiological conditions (PBS, pH 7.4). We studied by transmission electron microscopy the effect of the drug 4'-iodo-4'-deoxydoxorubicin (I-DOX) on the in vitro assembly of TTR Leu55Pro fibrils by following fibril growth over a 15 day period. Our results showed that I-DOX at a concentration of 10-5 M/100 microg fibrils does not inhibit fibril formation in up to 10 days since fibrils identical to the ones present in the untreated sample were observed. However, after 15 days of treatment, only round particles, resembling soluble native TTR, were observed. We also tested the ability of tetracyclines and nitrophenols to interfere with amyloid fibril formation for 17 days; the group of compounds tested showed fibril disruption activity to different extents: doxycycline and 2,4-dinitrophenol resulted in complete disaggregation of fibrils. The species generated upon I-DOX and tetracyclines treatments were nontoxic, as revealed by the lack of significant caspase-3 activation on a Schwannoma cell line, making them potential therapeutic drugs in TTR-related and other amyloidosis.