unnatural and as a result novel over their non enriched counterparts all percentages given for the amount of deuterium present are mole percentages it can be quite difficult in the laboratory to achieve 100 deuteration at any one site of a lab scale amount of compound e g milligram or greater when 100 deuteration is recited or a deuterium atom is specifically shown in a structure it is assumed that a small percentage of hydrogen may still be present deuterium enriched can be achieved by either exchanging protons with deuterium or by synthesizing the molecule with enriched starting materials the present invention provides deuterium enriched lenalidomide or a pharmaceutically acceptable salt thereof there are thirteen hydrogen atoms in the lenalidomide portion of lenalidomide as show by variables r 1 r 13 in formula i below the hydrogens present on lenalidomide have different capacities for exchange with deuterium hydrogen atoms r 1 r 3 are easily exchangeable under physiological conditions and if replaced by deuterium atoms it is expected that they will readily exchange for protons after administration to a patient hydrogen atoms r 9 and r 12 r 13 may be exchanged for deuterium atoms by the action of a suitable base such as t buok t buod the remaining hydrogen atoms are not easily exchangeable for deuterium atoms however deuterium atoms at the remaining positions may be incorporated by the use of deuterated starting materials or intermediates during the construction of lenalidomide the present invention is based on increasing the amount of deuterium present in lenalidomide above its natural abundance this increasing is called enrichment or deuterium enrichment if not specifically noted the percentage of enrichment refers to the percentage of deuterium present in the compound mixture.