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Preclinical evidence indicates that gene transfer to the dorsal root ganglion (DRG) using replication defective herpes simplex virus (HSV)-based vectors can reduce pain-related behavior in animal models of pain. This clinical trial was carried out to assess the safety and explore the potential efficacy of this approach in humans.
We conducted a multicenter, dose-escalation, Phase I clinical trial of NP2, a replication defective HSV-based vector expressing human preproenkephalin (PENK) in subjects with intractable focal pain caused by cancer. NP2 was injected intradermally into the dermatome(s) corresponding to the radicular distribution of pain. The primary outcome was safety. As secondary measures, efficacy of pain relief was assessed using a numeric rating scale (NRS), the Short Form McGill Pain Questionnaire (SF-MPQ) and concurrent opiate usage.
Ten subjects with moderate to severe intractable pain despite treatment with more than 200 mg/day of morphine (or equivalent) were enrolled into the study. Treatment was well tolerated with no study agent-related serious adverse events (SAE) observed at any point in the study. Subjects receiving the low dose of NP2 reported no substantive change in pain. Subjects in the middle and high dose cohorts reported pain relief as assessed by NRS and SF-MPQ.
Treatment of intractable pain with NP2 was well tolerated. There were no placebo controls in this relatively small study, but the dose-responsive analgesic effects suggest that NP2 may be effective in reducing pain and warrants further clinical investigation.
A significant limitation to the development of analgesic drugs is that off-target effects at doses below the maximal analgesic threshold restrict the ability to selectively interrupt nociceptive neurotransmission1. To address this limitation, we developed a series of replication defective HSV-based vectors to deliver gene expression cassettes directly to DRG neurons from skin inoculation 2, 3. The anatomically defined projection of DRG axons allows targeting of specific ganglia by injection into selected dermatomes. In preclinical studies, the release of anti-nociceptive peptides or inhibitory neurotransmitters in spinal dorsal horn from the central terminals of transduced DRG neurons effectively reduced pain-related behaviors in rodent models of inflammatory pain, neuropathic pain, and pain caused by cancer4-9.
The human PENK gene encodes for preproenkephalin, a precursor protein proteolytically cleaved to produce the endogenous opioid peptides met- and leu-enkephalin. In the spinal cord, enkephalin peptides inhibit pain signaling through actions at presynaptic opioid receptors located on central terminals of primary afferent nociceptors and postsynaptic opioid receptors on second order neurons involved in nociceptive neurotransmission10. HSV vectors expressing opioid peptides appear to be particularly effective in animal models of inflammatory and cancer pain4, 5, 8.
Because there have been no previous clinical trials of replication defective HSV as a gene transfer vector, we undertook this Phase 1 dose-escalation study of NP2, a PENK-expressing HSV vector, to assess the safety of the HSV-based gene transfer platform. As secondary outcome measures we evaluated the effect of vector-mediated delivery of PENK on pain, and on concurrent use of opiate medication.
The study was sponsored and implemented by Diamyd Incorporated, a subsidiary of Diamyd Medical AB. The protocol was reviewed by the NIH Recombinant DNA Advisory Committee (RAC), conducted in accordance with the FDA approved IND (#13613) in compliance with Good Clinical Practice (GCP) and listed on ClinicalTrials.gov as “Gene transfer for intractable pain: A phase I clinical trial to determine the maximum tolerable dose of a replication-defective herpes simplex virus type I (HSV-1) vector expressing human preproenkephalin (NP2) in patients with malignancies” (#NCT00804076).
NP2 is a replication defective HSV-based vector deleted for the essential Immediate Early (IE) genes ICP4 and ICP27 with a human cytomegalovirus IE promoter-driven human PENK gene expression cassette inserted into both ICP4 loci. The deletions of the ICP4 loci also remove the promoters for the ICP22 and ICP47 genes, rendering NP2 defective in expression of 4 HSV IE genes.
NP2 was propagated in an ICP4 and ICP27 complementing cell line created by sequentially transfecting Vero cells (ATCC Catalogue# CCL-81) with separate plasmids containing ICP4 and ICP27 and drug selection markers. Clonally derived cell lines were tested for complementing activity and stability prior to subsequent re-transfection and drug selection. The final isolated cell line, PHC11-54-1, provided stable propagation and amplification of NP2 and was expanded as a Master Cell Bank (MCB). The certified MCB was employed to produce the NP2 Master Virus Bank (MVB) and subsequently to produce the NP2 Clinical Product in compliance with FDA current Good Manufacturing Practices (cGMP). The MCB, MVB and clinical cGMP products each passed a thorough panel of safety and identity testing criteria. A Good Laboratory Practice (GLP) preclinical toxicology program was implemented in mice for up to 90 days after inoculation. No target organ of toxicity was identified at any time point. A comprehensive GLP preclinical biodistribution study established that quantifiable vector genomes were limited to the site of injection and the innervating DRG.
The NP2 clinical cGMP stock was produced by infecting 10-layer cell factories of MCB cells with NP2 MVB. The infected culture was harvested, salt treated, and clarified by centrifugation and filtration. Residual DNA was digested with endonuclease and the vector purified by ion exchange chromatography. The product was further purified and concentrated by tangential-flow filtration and recovered NP2 was sterile filtered, formulated, filled into cryovials and stored below -70° C until use. The final product was tested for safety (including absence of replication competent vector), identity, purity, titer, and enkephalin expression. A stability program was initiated to monitor NP2 product titer, enkephalin expression, pH, appearance, and sterility over the course of the trial.
Subjects 18 years of age or older with intractable pain due to primary or metastatic cancer that was moderate to severe in intensity (≥ 40 on a 100 mm visual analogue scale) despite: (a) stable analgesic treatment with at least 200 mg/day morphine or equivalent; or (b) having reached a stable maximum tolerated dose of narcotic or other analgesics that was less than 200 mg/day morphine or equivalent, were eligible for enrollment.
Trial subjects were excluded if they were pregnant and had to be able to reliably provide pain assessment. Subjects with a clinical diagnosis of active herpetic disease within 6 months of recruitment, or vaccination to prevent HSV infection were excluded. We also excluded patients with recent chemotherapy, radiation therapy, or a surgical stabilization procedure. Subjects with serious uncontrolled medical conditions other than malignancy, positive serology for HIV, hepatitis B or hepatitis C, severe anemia, uncontrolled coagulopathy or bleeding diathesis, or documented immunodeficiency were excluded.
This Phase 1 study was approved by the Institutional Review Boards (IRBs) of the University of Michigan and Chesapeake Research Review and was conducted in accordance with the principles of the Declaration of Helsinki. All subjects executed written informed consent prior to screening. Subjects that passed all inclusion and exclusion screening criteria had vector administered as 10 intradermal injections of approximately 100 μl each (total 1.0 ml) distributed within the dermatome(s) corresponding to the radicular distribution of the pain in a single session on study Day 0. Enrollment of the first cohort commenced with a dose of 1×107 plaque forming units (pfu). Subjects were monitored for four weeks and dosing sequentially escalated to the 1×108 and 1×109 pfu cohorts following approval by the Data and Safety Monitoring Committee (DSMC).
Subjects were observed for at least 12 hours after dosing before being discharged and were scheduled for re-examination at 1, 3, 7, 10, 14, 21 and 28 days post-dosing and monthly thereafter. Safety data examination included adverse events (AE), a full physical examination, vital signs, mucositis evaluation, and clinical laboratory analyses.
Secondary measures included the NRS for pain intensity and the SF-MPQ completed by the subject on study Day 0 prior to dosing and on post-dosing days 1, 3, 7, 10, 14, 21 and 28. An SF12 short form Health Survey and a complete Eastern Cooperative Oncology Group (ECOG) performance status were similarly completed on study Day 0 and day 28. Concurrent opiate analgesic usage was recorded on post-dosing study Days 1, 3, 7, 10, 14, 21 and 28.
Ten subjects were enrolled into the study. Four subjects received 1×107 pfu NP2; 3 subjects received 1×108 pfu NP2; and 3 subjects received 1×109 pfu NP2. Subject demographics are presented in Table 1. The mean pre-dosing NRS pain score was: 8.0 ± 0.6 for subjects in Cohort 1; 9.0 ± 0.57 for subjects in Cohort 2; and 7.7 ± 1.3 for subjects in Cohort 3.
No treatment-related SAEs were reported during the 4 month follow-up period. Of the 10 terminal cancer subjects enrolled, 8 completed the 28 day study and 4 subjects completed the 4 month follow-up. Four of the 6 discontinued subjects were withdrawn due to death (2 at the low dose, 1 at the mid dose, and 1 in the high dose cohort); one subject requested discontinuation after 105 days of follow up, and one subject discontinued for reasons that were not established. Six of the 10 enrolled subjects reported a total of 8 SAEs. These adverse events included: progression of malignancy, obstruction of the common bile duct, pneumonia, asthma, and pleuritic pain. None of the SAEs were judged to be related to the investigational product by the study site principal investigator or the DSMC.
Nine of the 10 enrolled subjects had one or more treatment-emergent AE (TEAE) during the 4 months following drug delivery. A total of 72 TEAEs were reported. The TEAEs that were deemed as possibly related to treatment were all mild in severity and resolved, including: transient injection site erythema and pruritus, and one subject who had a transient elevation in body temperature.
No subject seroconverted from anti-HSV antibody (Ab) negative at baseline to Ab positive at one or four months after inoculation with NP2, and none of the subjects who were anti-HSV Ab positive at enrollment showed any increase in anti-HSV Ab titer after receiving NP2. No HSV DNA was detected by qPCR in the blood or urine of subjects at 1, 7, or 14 days following NP2 dosing.
The principal secondary outcome of this trial was the numeric rating scale (NRS) of pain. Because of the terminal nature of subjects underlying cancer, by 2 months post-dosing there was only 1 patient surviving in the 107 pfu cohort, and 2 each in the 108 and 109 cohorts. By 3 months after dosing, there was only 1 patient remaining in the 107 and in the 108 cohorts. Therefore, reporting of secondary outcomes are limited to the 8 patients who remained in the study 28 days post-dosing.
Subjects in the lowest dose cohort (107 pfu NP2) did not report substantial changes in NRS over the 28 days following intradermal dosing. Subjects receiving 108 pfu NP2 reported an average reduction in NRS pain values at Day 1 to approximately 50% predosing values. Thereafter, the 108 pfu cohort subjects’ average NRS values continued to decrease to less than 20% of pre-dosing values at 7 and 14 days, after which the NRS began to rise though not back to baseline levels. Subjects in the 109 pfu NP2 cohort showed a similar reduction in average NRS pain scores to approximately 50% of baseline at day 1 that continued to decline to approximately 20% of baseline values by day 7 where the average NRS pain score remained below through the 28 day time point (Figure 1a). Although this was not a blinded study and there were no placebo controls, there was an apparent dose-response that could be measured as the percent maximal possible effect on the average NRS score from days 7 through 28 (Figure 1b). A similar trend was observed in section I of the subject reported SF-MPQ scores (Table 2) representing the sum of 15 individual scores (each between 0 and 3) in response to questions about the pain (sensory and affective). The dose of concurrent opiate medication that patients continued to take under the direction of their treating physician ranged broadly and it is difficult to draw conclusions on this parameter given the small number of patients in each cohort (Table 3).
Despite extensive basic research and clinical trials, chronic pain is a condition that often remains refractory to available treatments11-13. Innovative approaches have largely focused on the search for novel targets for analgesic drugs, based on insights gained from broad based investigations into the neurobiology of chronic pain14. Unfortunately, it has been difficult to identify amenable targets for systemically delivered small molecules that are located exclusively within nociceptive pathways. Not unlike more conventional analgesic agents15, 16, even drugs designed to interact with newly identified apparently “nociceptive specific” targets such as the vanilloid receptor TRPV1 have been found to have biological roles unrelated to nociception that pose a significant constraint to their use17, 18.
HSV is a ubiquitous, naturally neurotropic virus spread by contact of skin or mucous membranes that establishes a persistent latent state in neurons of sensory ganglia. By the same cellular transduction mechanisms, replication defective HSV-based vectors delivered subcutaneously or intradermally transfer genetic material into DRG neurons in vivo19. The human PENK gene carried by NP2 encodes preproenkephalin, a precursor protein that is processed to produce 6 met-enkephalin and 1 leu-enkephalin moieties; endogenous opioid peptides that are the naturally occurring high-affinity ligands of the delta opioid receptor10. In this clinical trial the mechanism of action of the vector was not directly confirmed, but in animal studies the analgesic effects achieved by HSV-mediated expression of PENK are blocked by opioid receptor antagonists naloxone and intrathecal naltrexone 4-6 suggesting a site of action at spinal opioid receptors, and in vitro biological effects of vector-produced enkephalins are blocked by 10 pM naltrindole 20 consistent with in vitro activity at the delta opioid receptor. While met-enkephalin has a higher affinity for the delta compared to the mu opioid receptor 21 it is certainly possible that the analgesic effects observed are mediated through mu as well as delta opioid receptors.
Conceptually, HSV-mediated delivery of PENK represents a logical extension of the technique of delivering opiate drugs by intrathecal infusion22 to maximize effects achieved at the spinal level while minimizing systemic side effects. Following delivery vector particles are taken up by nerve terminals in the skin and transported to the DRG where the circularized vector genome establishes a persistent state as an intranuclear episomal element. Vector derived expression of human PENK in DRG following skin inoculation of HSV vectors in rodents has been confirmed by in situ hybridization, RT-PCR, radioimmunoassay and immunocytochemistry 4, 23. While we were not able to study vector biodistribution in this clinical trial, preclinical studies indicate that replication defective vector genomes are constrained to the injection site and related DRG. This was confirmed by a preclinical GLP biodistribution study of NP2, in which 16 different tissues were examined between 1 and 91 days after inoculation.
Other gene transfer approaches including intrathecal injection of adenovirus24, adeno-associated virus25, or naked plasmids26 have been demonstrated to reduce pain-related behaviors in animal models of pain, but none of these approaches have yet been brought to a human clinical trial. In preclinical animal studies, skin inoculation of HSV vectors expressing PENK reduce acute hyperalgesic responses27, and reduce pain-related behaviors in models of arthritis28, formalin injection4, peripheral nerve damage6 and bone cancer5. Because this was the first human trial employing HSV vectors to achieve gene transfer, we elected to carry out the phase 1 clinical trial for safety and dose-finding in patients with pain caused by cancer.
The safety profile observed in this study was not unanticipated. Oncolytic recombinant HSV-1 viruses without transgenes that are intended to kill malignant cells by limited replication have been injected directly into tumors in brain, liver, and skin in more than 200 subjects to date with no reported test agent-related SAEs29-33. Replication-competent HSV recombinants have also been examined in clinical trials as potential vaccines against genital herpes. While these approaches have in some cases generated quite high anti-HSV antibody levels (though not effectively preventing HSV infection), no drug related SAEs have been observed34-36.
This Phase I clinical trial primarily addressed the question of whether intradermal delivery of NP2 to skin would prove to be safe and well tolerated by subjects. The small number of patients and the absence of placebo controls warrant circumspect interpretation of the secondary outcome measures. But the observation that subjects in the low dose cohort had little change in the NRS or SF-MPQ while subjects in the higher dose cohorts reported substantial reduction in NRS and improvement in SF-MPQ is encouraging. Based partially upon these results, Diamyd has initiated a randomized, double- blind placebo-control Phase II clinical trial in a similar patient population (A phase II, randomized, double blind, placebo-controlled, multicenter study to investigate the impact of NP2 in subjects with intractable pain due to malignancy; ClinicalTrials.gov #NCT01291901).
While pain associated with cancer is a significant clinical problem with unmet medical need37, the potential utility of HSV-mediated gene transfer to the DRG from skin inoculation is not limited to treatment of cancer pain. The recombinant replication defective HSV approach represents a platform technology - nerve targeting drug delivery system (NTDDS) - that can be used to deliver and express any one of a number of genes in the nervous system. A related NTDDS vector, NG2 that expresses human glutamic acid decarboxylase (GAD) to effect the release the inhibitory neurotransmitter γ aminobutyric acid reduces pain-related behaviors in preclinical models of neuropathic pain from nerve injury38 and diabetes (European Journal of Pain, in press). Clinical trials of NTTDS vectors in pain have the advantage that the biological effect of the transgene product can be assessed continuously in real time. NTDDS gene transfer to the DRG to express neurotrophins locally prevents the progression of polyneuropathy in relevant preclinical models39-41 suggesting that the NTDDS platform may be used to treat degenerative polyneuropathies as well.
We acknowledge the assistance of Kevin Weatherwax, Susan Urba, Srinivas Chiravuri, Gayle Estep, Heidi L'Esperance and Mary Orr at the University of Michigan, Samantha Amaba at AdvancedPharma CR, William Goins and Shaohua Huang at the University of Pittsburgh, and Joseph Dunn at Pain Research of Oregon. DJF and MM were supported in part by NIH grants DK044935, NS038850, and patients enrolled at the University of Michigan were supported in part by UL1RR024986. JCG was supported in part by DK044935.