This work addresses the problem of precoding optimization for a multi-user and multi-target ISAC system from a communication-centric perspective. Specifically, the paper considers a scenario where a multiple-antenna base station (BS) serves several single-antenna communication user equipments (UE) while also steering beams for target detection. The objective is to maximize the signal-to-interference plus noise ratio (SINR) subject to power and radio-sensing constraints. The problem is challenging due to its non-convexity. To address this, we propose a sub-optimal solution by zero-forcing (ZF) multi-user interference. A closed-form solution is derived, which efficiently computes the precoder with low computational cost. Simulation results validate the effectiveness of our proposed precoder design which outperforms previous precoder design methods for ISAC.