Btk inhibitors

We really have some excellent drugs for killing the circulating cells in CLL. Even chlorambucil is good at lowering the white count and fludarabine is excellent at it. I have seen FCR restore a white count to normal after a single course. The problem with CLL is that the peripheral blood is not where the action is. Although CLL cells are long lived and only die slowly, the real problem is that they are inexorably proliferating. This proliferation takes place in proliferation centers in the lymph nodes, spleen and bone marrow. Proliferation occurs because signals to proliferate are sent through messenger pathways from surface stimulation. In CLL the main signals come through the B-cell receptor (BCR). In unmutated CLL the BCR signaling works much better than in mutated CLL, which is why the former has a worse prognosis.

When the BCR is stimulated signaling through syk stimulates a host of other pathways that activate transcription factors in the nucleus. Chief among these pathways are the ERK, PI3K, and the NFkappaB pathways. It would be possible to develop drugs to inhibit these pathways, of course, but since they are used by every cell in the body, the effect would be a sort of universal toxicity as all cell types would be affected. Luckily, nature points the way.

Immunodeficiency diseases are great disasters for the patient. You probably have heard of the boys in the bubbles; young boys who have to live in plastic tents with sterile air pumped in, because they are susceptible to every germ. The only remedies for such diseases are a bone marrow transplant or else gene therapy. But there is a type of immunodeficiency that is easier to treat. Congenital hypogammaglobulinemia can be treated by regular infusions of gamma globulin. The type that occurs in young boys is called Bruton’s X-linked hypogammaglobulemia and is cause by a mutation in the gene for Bruton’s tyrosine kinase (Btk). Although this gene is expressed in many hematopoietic cells, when the gene is knocked out in mice it seems to be only effectively used by B cells; it doesn’t affect other cells in the blood and the only deficiency is in the immunoglobulins.

Since Gleevec, everybody is into tyrosine kinase inhibitors. Ideally they should be highly selective, only inhibiting specific tyrosine kinases, and that tyrosine kinase should have a narrow applicability. The orally available inhibitor PCI-32765 has been shown to be very useful at inhibiting the type of large diffuse B-cell lymphomas that are driven by BCR signaling. When used in dogs with aggressive B-cell lymphomas it produced remarkable clinical effects. Fairly specific kinase inhibitors have already been used in CLL with good effect; the syk inhibitor, fostamatinib, and the PI3K-delta specific inhibitor CAL-101. An article has been pre-published in Blood on the pre-clinical effects of PCI-32765.

They first showed that although BTK protein is variably expressed in CLL it does not correlate with prognostic markers. However, BTK mRNA was only modestly variable though significantly higher than that found in normal B cells.. They then showed that PCI-32765 induces cytotoxicity in CLL cells irrespective of IGVH mutations and FISH. However, the cytotoxicity is dependent on the Caspase pathway. In other words it is caused by apoptosis. PCI-32765 does not kill T cells but it alters their production of cytokines, inhibiting the production of IL-6.IL-10 and TNF-alfa after CD3 ligation. This suggests that PCI-32765 can inhibit other tyrosine kinases than Btk.

The authors also showed that in some CLL patients there is constitutive phosphorylation of of Btk independent of src activation. PCI-32765 inhibits both constitutive and activation induced phosphorylation of Btk as well as downstream activation of the PI3K, MAPK and NFkappaB pathways.

PCI-32765 also inhibits proliferation of CLL cells when stimulated by CpG oligonucleotides, (as a representative of Toll-like receptor signaling), and by CD40L and BAFF signaling, TNF-alfa, IL-6 and IL-4 signaling and by fibronectin signaling. The effect of PCI-32765 in squashing proliferation was not inhibited by co-incubation with Hs5 stromal cells. The overall effect of these experiments is to demonstrate that PCI-32765 has an inhibitory effect on proliferation centers in the tissues.

The effect of an early trial of PCI-32765 was reported at ASH last year (Abstract 57). 30 CLL/SLL patients were entered into the study. 84% of participants were man with a mean age of 68 (44-82).. The median number of prior therapies was 3 (1-4). Treatment was well tolerated; most of the 10 grade 3 side effects were thought to be unrelated to treatment. There was one case of grade 3 viral adenitis and one case of grade 2 viral infection related to the study medication. Btk inhibition was demonstrated. Of 14 patients available for evaluation of response, there was one CR, 8PRs, 4 SDs and one PD.

A second Btk inhibitor has been described. Avila Therapeutics presented a summary of the results of the first-in-human study AVL-292-001, at the Keystone Symposium on molecular and cell biology: evolving approaches for early-stage drug discovery. The study of AVL-292-001 was a double-blind, placebo-controlled, single ascending dose trial in healthy volunteers, in this study favourable safety, tolerability and pharmacokinetics were shown. About 50 healthy voluntary took in two studies, and based on these early and encouraging results, Avila are actively moving forward the clinical development of AVL-292 for treatment of B cell tumors, expecting to initiate a phase 1B study in CLL in mid-2011.