長年の研究が実を結び、画期的な新薬が開発された。___、その薬には重い副作用があることが後に判明した。

'しかるに' is a formal conjunction used to introduce a fact or situation that is contrary to what would be expected from the preceding statement. It's similar to 'however' or 'and yet'. In this context, the development of a new drug is a positive event, so the discovery of serious side effects is an unexpected, contrary outcome. 'ゆえに' and 'したがって' indicate a logical consequence, which doesn't fit the contrast. 'ちなみに' is used to add supplementary information.