The c-Myc (Myc) oncoprotein is a high-value therapeutic target given that it is 
deregulated in multiple types of cancer. However, potent small molecule 
inhibitors of Myc have been difficult to identify, particularly those whose 
mechanism relies on blocking the association between Myc and its obligate 
heterodimerization partner, Max. We have recently reported a structure-activity 
relationship study of one such small molecule, 10074-G5, and generated an 
analog, JY-3-094, with significantly improved ability to prevent or disrupt the 
association between recombinant Myc and Max proteins. However, JY-3094 
penetrates cells poorly. Here, we show that esterification of a critical 
para-carboxylic acid function of JY-3-094 by various blocking groups 
significantly improves cellular uptake although it impairs the ability to 
disrupt Myc-Max association in vitro. These pro-drugs are highly concentrated 
within cells where JY-3-094 is then generated by the action of esterases. 
However, the pro-drugs are also variably susceptible to extracellular esterases, 
which can deplete extracellular reservoirs. Furthermore, while JY-3-094 is 
retained by cells for long periods of time, much of it is compartmentalized 
within the cytoplasm in a form that appears to be less available to interact 
with Myc. Our results suggest that persistently high extracellular levels of 
pro-drug, without excessive susceptibility to extracellular esterases, are 
critical to establishing and maintaining intracellular levels of JY-3-094 that 
are sufficient to provide for long-term inhibition of Myc-Max association. 
Analogs of JY-3-094 appear to represent promising small molecule Myc inhibitors 
that warrant further optimization.