Cancer-Selective Targeting of the NF-κB Survival Pathway in Multiple Myeloma with the GADD45β/MKK7 Inhibitor, DTP3

The NF-κB transcription factor pathway is aberrantly activated in multiple myeloma (MM) and many other cancers, where it promotes malignancy by upregulating survival genes, thus providing a compelling rationale for therapeutically targeting this pathway in MM. However, despite aggressive efforts to develop a specific NF-κB or IκB kinase (IKK)β inhibitor, no such inhibitor has been approved, due to the preclusive toxicities associated with the general suppression of NF-κB. As a key pathogenetic activity of NF-κB in MM is to block apoptosis through the induction of target genes, an attractive alternative to globally inhibiting NF-κB would be to therapeutically target the non-redundant, cancer-specific downstream effectors of the NF-κB survival pathway.

Recently, we identified the interaction between the NF-κB-regulated antiapoptotic factor, GADD45β, and the JNK kinase, MKK7, as a pathogenically critical and cancer cell-restricted survival module downstream of NF-κB and novel therapeutic target in MM. Further, we developed a D-tripeptide inhibitor of the GADD45β/MKK7 complex, DTP3, which effectively kills MM cells by inducing MKK7/JNK-dependent apoptosis and, importantly, is not toxic to normal tissues. Due to this cancer-cell specificity, DTP3 has similar anti-cancer efficacy to bortezomib, but more than 100-fold higher cancer-cell specificity in patient MM cells, ex vivo. DTP3 also displays potent and cancer-selective activity against MM in preclinical animal models, with no apparent side-effects, and far greater therapeutic index than existing treatments. DTP3 further displays synergistic activity with conventional MM therapies, such as bortezomib, suggesting a clinical utility as frontline combination therapy. Additionally, it retains therapeutic efficacy in MM cells that are resistant to most common MM treatments, suggesting further clinical utility.

We currently aim to conduct a phase I/IIa clinical trial of DTP3 in MM to deliver clinical Proof of Concept for a cancer-selective NF-κB-targeting strategy as a highly effective novel therapy. This first-in-man study will commence in late 2015.

We report here the results from the regulatory pharmacodynamics (PD), safety pharmacology, pharmacokinetic (PK), and toxicology studies.

28-day intravenous (i.v.) repeat dose toxicology studies in rat and dog demonstrate that DTP3 is well tolerated, with no significant target organs of toxicity at up to 17 times the optimal exposure in mouse efficacy models. Toxicokinetic (TK) analyses indicate that DTP3 does not accumulate on repeat dosing. Safety pharmacology studies indicate no adverse effect on the central nervous, cardiovascular or respiratory systems. In an Ames assay, DTP3 was not mutagenic.

In the rat, i.v. DTP3 rapidly and extensively distributes to tissues, does not pass the blood-brain barrier, and is readily eliminated in urine (30%) and faeces (60%). No major metabolites have been identified.

In vitro, DTP3 did not inhibit or induce major human cytochrome P450 isoforms, suggesting no potential for drug interactions via P450. In vitro, DTP3 is neither a substrate for nor inhibitor of P-gp or BCRP, nor is it a substrate or inhibitor of most transporters evaluated. However, DTP3 is a substrate for the uptake transporters, MATE1, MATE2-K and OATP1B3, with some inhibitory potential for MATE1 and MATE2-K at high concentrations.

PD studies in a mouse xenograft model show that i.v. bolus injection of at least 10 mg/kg of DTP3 over 2 weeks, daily, every other day, or every 3 days, is highly effective in causing complete tumour regression. The PK and TK evaluation of DTP3 in rat and dog identified long plasma half-lives, modelled into a half-life of 20-24 hr in man. On these bases, we have proposed an initial clinical dosing regimen of DTP3 of three times per week i.v., testing doses from 0.5 to 20 mg/kg. The data also support a subcutaneous route of administration, and an i.v. bolus regimen of twice per week.

Collectively, the preclinical package demonstrates the outstanding selective pharmacology and efficacy of DTP3, combined with excellent i.v. tolerability, wide therapeutic window, and favourable PK profile, and supports progression into clinical development.

A companion biomarker programme has also been developed in order to inform patient stratification, demonstrate pathway-specific PD activity and proof of mechanism, and so maximise the chance of a positive clinical response.