neoplastic (adj.): of, pertaining to, or having the characteristics of tumors or other useless newly grown tissue
pleonastic (adj.): of, pertaining to, or having the characteristics of superfluous iteration or repetition in speech or writing

I don’t have much to say about this pair of words, actually, other than this: Never in my many years speaking English have I seen two four-syllable words with wholly different meanings wherein two consonant clusters can be interchanged. I think it’s truly remarkable that these two long, contrived words both exist. We’re not talking about a pairing like “vase” and “save,” where the limited number of phonologically acceptable letter combinations in a single-syllable, four-letter word means that some reciprocity is statistically (more or less) inevitable. There are, I’m sure, dozens of examples of that type (rite/tire and late/tale come to mind). But in what other instance does a ten-letter word demonstrate this kind of pairing? It’s so rare as to seem almost an error, a Spoonerism or malaprop of some kind. If you can think of any other examples that fit into this mold, please, gentle reader, let me know.