REGN4336

This study is researching an investigational drug called REGN4336 both alone or together with another investigational drug called REGN5678. The study is focused on participants with previously treated metastatic prostate cancer.
The main purpose of the study is to look at the safety, tolerability (how the body reacts to the drug) and effectiveness (how well the drug works to shrink tumors) of REGN4336 when given in combination with REGN5678.
The study has 2 parts. The purpose of Part 1 is to determine a safe dose(s) of REGN4336 to be given alone or in combination with REGN5678. Part 1 is known as the "dose escalation" phase.
The purpose of Part 2, (known as the "dose expansion" phase), is to use the doses of REGN4336 and REGN5678 selected in Part 1 to further test how well the combination treatment with REGN4336 and REGN5678 works to shrink tumors.
The study is looking at several other research questions, including:
* What side effects may happen from taking REGN4336 alone or in combination with REGN5678
* How well does REGN4336 in combination with REGN5678 reduce tumor size
* How much REGN4336 is in the blood at different times when it is given alone or in combination with REGN5678
* Does the body make antibodies against the study drugs (REGN4336 or REGN5678)