Lesson 7 – Peak Plasma PS ASO Concentration-related Toxicities

Introduction

Like desired effects, toxic effects are dose-dependent. While most chronic toxicities correlate with the total cumulative dose, some toxicities are related to the peak drug concentration in an organ.

I have argued that plasma should be considered an organ, albeit liquid. One role plasma plays is to integrate a number of stress responsive pathways.

• The clotting cascade
• The complement cascade
• The Kinin-kallikrein cascade
• Acute phase reactant cascades
• The angiotensinogen cascade

A variety of cells in blood and the vascular endothelium play the key roles of binding to various members of these pathways, including PS ASOs. One cell type that is abundant and plays a surface-activating role is platelets. Essentially all the protein factors involved in these cascades are made in the liver as pro-proteins that must undergo proteolytic cleavage to be activated and many are proteases themselves that activate the next step in the cascade.

Each of the systems above are designed to respond urgently to threats that range from bleeding to responding to infectious organisms to the management of blood pressure and they all interact and must be very tightly controlled.

Activation of these systems can also result in activation of adaptive immunity and the production of deleterious auto-antibodies.

PS ASOs bind to clotting factors

Though PS ASOs clearly bind to liver-derived vitamin K dependent clotting factors, we have never observed any deleterious consequences of binding to clotting factors.

PS ASOs bind to factor H in the complement pathway

Factor H is the key negative regulator of complement activation. It inhibits complement activation.

Factors H from different species vary and non-human primate factor H appears to have a higher affinity for PS ASOs than human factor H.

In NHPs in tox studies, at high doses, we observe complement activation in some animals. Early in the development of the technology, there are acute deaths of NHPs due to complement activation. Once the issue was identified, we learned how to manage this.

• Different strains or types of NHPs vary in their response to PS ASOs, with Mauritius NHPs being the most sensitive.

We have never observed any acute activation of complement or acute adverse events in humans.

Platelets, complement activation, chronic systemic inflammation and PS ASOs

In a tiny fraction of patients with very high chronic inflammation, such as TTR amyloidosis or familial chylomicronemia, after dosing for >4 months with 300mg/week S.C. with PS 2’-MOE gapmers, we saw acute reductions in platelets and one bleeding event.

Mechanism of thrombocytopenia

Though there are still elements that are not fully understood, the mechanism appears to be related to binding of PS ASOs to platelets and perhaps slight activation of complement and, in some patients, the development of anti-PS ASO anti-platelet antibodies.

The process is dependent on high peak plasma PS ASO concentrations. In essence, over a few months of repeated exposures to high PS ASO plasma concentrations, in patients with chronic systemic inflammation priming their responsiveness to immune stimuli like PS ASOs, some or all of these systems can be activated.

Response: Avoid very high plasma PS ASO concentrations.

• If we need to dose at 300mg or greater doses in humans, we divide (For example, two 150mg S.C. doses separated by 60 minutes.) to avoid any risk of this toxicity.

Conclusions

PS ASOs bind to both plasma proteins and blood cells. Binding to serum albumin and other plasma proteins assures that PS ASOs are not cleared rapidly in urine by filtration by the kidney. Despite the fact that PS ASOs bind to plasma albumin, PS ASOs do not bind at sites that bind small molecule drugs and, therefore, there are no drug-drug interactions due to albumin binding.

PS ASOs bind to liver-derived clotting factors, but effects on clotting are transient and have not been associated with clinically significant sequelae.

PS ASOs bind to complement factor H, a protein that inhibits complement activation. Though binding to factor H in non-human primates (NHPs) can be associated with complement activation and serious inflammatory effects, no complement activation in humans has been observed. We think that the affinity of PS ASOs for NHP factor H is greater than for human factor H.

The only significant peak plasma concentration-related event was observed with chronically administered (>4 months) at a dose of 300mg in patients with significant chronic systemic inflammation associated with their disease, such as familial chylomicronemia (FCS) or TTR amyloidosis. Rarely, in these patients, significant, sometimes severe reductions in platelet counts (thrombocytopenia) have been observed. The mechanism accounting for thrombocytopenia appears to be associated with PS ASO binding to platelets and the generation of anti-PS ASO-anti-platelet antibodies. To avoid this risk, we simply do not administer doses greater than 200-250mg.

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