11. Men and women of reproductive potential who are not using effective contraceptive methods. The effects of Plitidepsin on the developing human fetus at the recommended therapeutic dose are unknown. For this reason, women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry, for the duration of study participation and for 6 months after. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately. Also, if the wife or female partner of a male patient becomes pregnant during the study, the investigator must be notified promptly and the pregnant woman will be referred for appropriate follow-up with a High-risk Obstetrician.