It is challenging to elucidate the extrinsic effects of social, economic , policy or other substantive changes on the dynamics of  US drug approvals.
<meaning-changed> It is challenging to elucidate the extrinsic effects of social, economic , policy or other substantive changes on the dynamics of  US drug approvals.
It is challenging to elucidate the extrinsic effects of social, economic , policy or other substantive changes on the dynamics of changes in external influences (such as economic or policy) on the rate of US drug approvals.