The MTD will be defined as the last dose level that is successfully administered with a decision to escalate to the next level. If the decision not to escalate to the next level is made then a cohort that receives XTL 2125 at a dose half-way between the last tolerated dose and the non-tolerated dose may be enrolled at the discretion of the Sponsor. If this dose is successfully administered without violating the dose escalation rules, then this interim dose will be considered the MTD.