Preliminary results from a phase I study of substance P-saporin in terminal cancer patients with intractable pain.

191 Background: For 10-15% of cancer patients, pain cannot be controlled using existing therapies (Zech et al., Pain, 63, 65-76, 1995). SP-SAP is the first targeted toxin to undergo phase I testing for pain. SP-SAP (Wiley and Lappi, Neurosci Lett, 230, 97-100, 1997) covalently linkes substance P (SP), a neuromodulator that binds to NK1 receptors on laminae I and X of the dorsal horn, and saporin (SAP), a ribosomal toxin that ablates cells that express NK1 and endocytose the toxin. Intrathecal injections of SP-SAP in rats reduce chemically or thermally induced pain, thermal hyperalgesia, and mechanical allodynia, with no signs of motor, sensory, or behavioral dysfunction (Mantyh et al., Science, 278, 275-9, 1997; Nichols et al., Science, 286, 1558-61, 1999; Suzuki et al., Nature Neurosci, 5, 139-26, 2002; Vierck et al., Neurosci, 199, 223-32, 2003). In canines, intrathecal injections into the lumbar region – 45 mcg for 10-15 kg (Allen et al., Toxicol Sci, 91, 286-98, 2006) or 15 mcg for 8-12 kg (Wiese et al., Anesthesiol, 119, 1163-77, 2013) – produce no neurological, behavioral, or histological deficits. A double-blinded RCT of SP-SAP for the treatment of bone pain in canines (Brown and Agnello, Anesthesiol, 199, 1178-85, 2013) showed that a single injection of 60 mcg (above 30 kg) or 40 mcg (below 30 kg) significantly reduced pain as measured by time to owner unblinding (p=0.002) or number of canines unblended (p=0.001). Methods: A single intrathecal injection of SP-SAP is made at the L5-S1 interspace of cancer patients with intractable sub-umbilical pain. Single subject cohorts are at doses of 1, 2, 4, 8, 16, 32, 64, and 90 mcg. Primary outcomes are assessed by pain survey (VAS “bothersome pain”, VAS “pain intensity”, ODI, EQ-5D, and BDI) and medication use log. Patients are monitored for toxicity, neurological and cardiac abnormalities, and SP-SAP antibodies for 6 months. Results: This report presents the results from the first three subjects of the phase I study. No evidence of toxicity or neurological or cardiac abnormalities were found. No clear evidence for pain reduction was observed. Conclusions: At existing doses (< 4 mcg), SP-SAP appears safe for human use. The study is continuing with dose escalation. Clinical trial information: NCT02036281.