Patent Description:
Many designs of injector pen are known. Typically the pen has cylindrical pen body and a dose selector. In a dose setting phase of operation the user moves the dose selector axially and/or rotationally relative to the pen body to a position that determines the dose for delivery, then in a drug delivery phase of operation the user pushes the dose selector axially into the pen body to deliver the dose. The pen further comprises means for attaching a drug cartridge to the pen; and a piston rod, which moves along the axis of the pen to act on the piston of the drug cartridge. In the drug delivery phase the proximal end of the pen is pressed, e.g. by the user's thumb, to push the dose selector into the pen body. During drug delivery, a drive mechanism within the pen body converts the movement of the dose selector into axial movement of the piston rod through a distance suitable to deliver the required dose from the cartridge. The distance moved by the piston rod is thus determined by the distance moved by the dose selector but the two distances need not be equal. On each use of the pen to deliver the drug from the same cartridge, the piston rod advances further along the axis. The piston rod does not move axially during the dose setting phase.

Published international patent application <CIT> discloses an injector pen of this kind, in which a driver is in threaded engagement with a piston rod, such that rotation of the driver causes the piston rod to move axially relative to a piston rod guide in the pen housing. A first circular ratchet constrains the driver to rotate only in a direction that drives the piston rod to advance. A reset ring is rotationally locked to the driver and is coupled via a second circular ratchet to a dose knob. The user can set a variable dose by screwing the dose knob along a helical thread to a position that corresponds to the desired dose. During dose setting, the reset ring follows the axial movement of the dose knob but does not rotate. During dose delivery, the user pushes the dose knob back along the helical thread and the second circular ratchet causes the reset ring to rotate with the dose knob, thereby causing rotation of the driver and axial movement of the piston rod to expel drug from a cartridge.

In many cases, it is desirable that the dose of the drug to be delivered by an injector pen should not be freely selectable by the user but should be a fixed volume, determined in advance to match a prescribed dose. The user should be prevented from setting and delivering only a partial dose.

Because the injector pen can deliver multiple doses of drug from a single cartridge, after repeated uses the cartridge may reach a state where it has less than a full dose of the drug remaining. In that case, regulations typically require that the pen should be automatically locked to prevent further use. This function is referred to as "last dose lock-out".

The invention provides a fixed dose injector pen, as defined in claim <NUM>.

Preferred but non-essential features of the invention are defined in the dependent claims.

In this specification, the word "drug" is used to describe any fluid substance that is to be delivered by the pen in measured doses. It will typically be a biologically active substance that is injected into the body of a human or animal subject, e.g. for medicinal or cosmetic purposes. However, the invention could be used in other applications where it is desired to dispense fixed quantities of a substance.

An injector pen according to a preferred embodiment of the injection is illustrated in <FIG>. It comprises a generally cylindrical, hollow pen body <NUM> centred on a longitudinal axis <NUM> of the pen. An injector element <NUM> is mounted in the pen body <NUM> and is configured to slide axially relative to the pen body <NUM> for setting or delivering a dose of a drug. Relative rotation between the pen body <NUM> and the injector element <NUM> is prevented by complementary features such as a pair of flats or a key and keyway (not illustrated). A proximal end of the injector element <NUM> projects from a proximal end of the pen body <NUM> to form an injector button <NUM> that can be gripped manually by the user to withdraw it from the pen body <NUM> (<FIG>) or push it back into the pen body <NUM> (<FIG>).

A drug cartridge <NUM> is mounted on a distal end of the pen body <NUM>. The drug cartridge <NUM> comprises a chamber <NUM> that contains multiple doses of a drug for injection into a patient. The drug cartridge <NUM> is enclosed by a cover <NUM> that provides connection means for retaining the drug cartridge <NUM> in the pen body <NUM>. The cover <NUM> further comprises a thread <NUM> or other means for mounting a hypodermic needle (not illustrated) in fluid communication with the chamber <NUM>. A piston <NUM> in the chamber <NUM> can be pushed along the axis <NUM> of the pen to force doses of the drug out of the chamber <NUM> via the needle. The distal end of the pen is covered by a removable pen cap <NUM> when not in use.

The injector pen comprises a piston rod <NUM> that lies along the axis <NUM>, partly within the pen body <NUM> and partly extending from the distal end of the pen body <NUM> to penetrate the chamber <NUM> of an attached drug cartridge <NUM>. By moving along the axis <NUM> in a direction from the proximal end towards the distal end of the pen body (the "first axial direction"), the piston rod <NUM> pushes the piston <NUM> along the axis <NUM> to deliver a dose of the drug from the chamber <NUM>.

In this embodiment of the invention, the piston rod <NUM> is solid in cross-section and carries a helical thread <NUM> on its outer surface. The piston rod <NUM> also comprises an opposing pair of flats <NUM> extending along its length, as a result of which the thread <NUM> is discontinuous.

A piston rod guide <NUM> is secured within the pen body <NUM> such that it cannot rotate. The piston rod guide <NUM> is mounted concentrically with the piston rod <NUM> and comprises an opposing pair of internal flats <NUM> that engage the flats <NUM> of the piston rod <NUM> to allow the piston rod <NUM> to slide through the piston rod guide <NUM> but to prevent the piston rod <NUM> rotating (<FIG>). It will be apparent that methods of engagement other than flats <NUM>,<NUM> could be used to achieve a similar sliding but non-rotating coupling between the piston rod <NUM> and the piston rod guide <NUM>, e.g. a pin on the guide <NUM> acting in a keyway of the piston rod <NUM>; or the piston rod <NUM> and the piston rod guide <NUM> having alternative complementary, non-circular cross sections.

Axial movement of the piston rod <NUM> is driven by a piston rod driver <NUM> that is mounted coaxially in the pen body <NUM> so as to be capable of rotation but not axial translation. A bore in the piston rod driver <NUM> comprises an internal thread <NUM> that engages the external thread <NUM> on the piston rod <NUM>. Because the piston rod <NUM> is prevented from rotating by the piston rod guide <NUM>, rotation of the piston rod driver <NUM> about the threaded coupling drives the piston rod <NUM> to slide along the axis <NUM>. A first circular ratchet <NUM> (<FIG>) permits rotation of the piston rod driver <NUM> only in a first rotary direction, which is the direction that causes the piston rod <NUM> to advance in the first axial direction and deliver a dose of the drug from the cartridge <NUM>. In the illustrated embodiment, the first circular ratchet <NUM> comprises ratchet arms <NUM> and a circular pawl <NUM>. The ratchet arms <NUM> are formed integrally with the piston rod driver <NUM> and extend outwards from it. The pawl <NUM> is formed as a discrete element and fixedly mounted in the pen body <NUM>. It is apparent that the ratchet <NUM> could take other forms: for example, the ratchet arms <NUM> could be provided by a discrete element attached to the piston rod driver <NUM> and/or the circular pawl <NUM> could be formed integrally with the pen body <NUM> or the piston rod guide <NUM>.

A cylindrical transmission element <NUM> is mounted concentrically in the pen body <NUM>, outside the piston rod driver <NUM> and inside the injector element <NUM>. The transmission element <NUM> is able to rotate about the axis <NUM> but is constrained so it cannot move along the axis <NUM>. A male helical thread <NUM> projects from an outer cylindrical surface of the transmission element <NUM> and engages a female helical groove <NUM> in an inner cylindrical surface of the injector element <NUM>. The helical coupling formed by the thread <NUM> and the groove <NUM> is non-self-locking so that sliding the injector element <NUM> axially in either direction causes rotation of the transmission element <NUM>. It is apparent that the helical coupling can be achieved in alternative ways, e.g. by interchanging the male thread <NUM> and the female groove <NUM>, and that the male component need not be a complete thread but could comprise small projections that travel along the groove <NUM>. In the illustrated embodiment the thread <NUM> and the groove <NUM> have two starts but a different number of starts could be chosen.

A second circular ratchet <NUM> is mounted between the transmission element <NUM> and the piston rod driver <NUM> for selectively transmitting the rotation of the transmission element <NUM> to the piston rod driver <NUM>. As seen in <FIG>, the second circular ratchet <NUM> comprises two resilient ratchet arms <NUM> that extend outwards from the piston rod driver <NUM> and a corresponding pair of diametrically opposed, axial grooves <NUM> formed in an inner cylindrical surface <NUM> of the transmission element <NUM>. In the illustrated embodiment of the invention, the ratchet arms <NUM> are part of a discrete ratchet element <NUM> that is fixed to the piston rod driver <NUM> but it is apparent that in alternative embodiments they could be formed integrally with the piston rod driver <NUM>. Each groove <NUM> has an asymmetric cross section, with one wall at a shallow angle and the opposing wall at a steep angle, and the tip of each ratchet arm <NUM> has a complementary shape that can lodge against the steeper wall of the groove <NUM> during rotation of the transmission element <NUM> in the first rotary direction, but can ride over the shallow wall of the groove <NUM> during rotation of the transmission element <NUM> in the opposite direction (hereafter the "second rotary direction"). It is apparent that axially extending grooves <NUM> are not the only recess-like structure that is capable of selectively engaging the ratchet arms <NUM>: for example, the inner cylindrical surface <NUM> could be provided with suitably shaped pits or circumferentially-facing steps to achieve a similar function.

When the injector element <NUM> is withdrawn from the pen body <NUM> in a dose setting phase of operation, it causes the transmission element <NUM> to rotate in the second rotary direction. Then the tips of the ratchet arms <NUM> ride over the ratchet grooves <NUM> and do not transmit the rotation to the piston rod driver <NUM>. The first circular ratchet <NUM> also prevents rotation of the piston rod driver <NUM> in the second rotary direction that might result from frictional contact between the ratchet arms <NUM> and the inner cylindrical surface <NUM>.

When the injector element <NUM> is pushed into the pen body <NUM> in a dose delivery phase of operation, it causes the transmission element <NUM> to rotate in the first rotary direction. Then the ratchet grooves <NUM> can firmly engage the tips of the ratchet arms <NUM> and transmit the rotation to the piston rod driver <NUM>, which in turn drives the piston rod <NUM> to advance along the axis <NUM> and deliver a dose from the drug cartridge <NUM>.

It should be noted that the ratchet arms <NUM> can engage the grooves <NUM> only at certain engagement positions around the circumference of the transmission element <NUM>. In between those engagement positions, the ratchet arms <NUM> slide freely over the inner cylindrical surface <NUM> of the transmission element <NUM>, when rotated in either direction.

When the injector pen has completed delivery of a dose of the drug, the injector element <NUM> has been pushed fully into the pen body <NUM> to reach its final position, as shown in <FIG>, and the transmission element <NUM> has been rotated in the first rotary direction with the second circular ratchet <NUM> engaged. If the injector element <NUM> is now withdrawn to set a new dose for delivery, it causes the transmission element <NUM> to rotate in the second rotary direction with the ratchet arms <NUM> sliding over its inner surface <NUM>. It can be seen from <FIG> that the transmission element <NUM> needs to rotate through <NUM>° in the second rotary direction before the ratchet arms <NUM> can once again engage in the ratchet grooves <NUM>. If the rotation is reversed before reaching <NUM>°, the arms <NUM> will not engage and the rotation will not be transferred to the piston rod driver. If the dose-setting rotation in the second rotary direction exceeds <NUM>°, the ratchet arms <NUM> will first ride over the grooves <NUM> and continue sliding over the inner surface <NUM> until, when the direction is subsequently reversed to deliver a drug, the arms <NUM> slide back and re-engage the grooves <NUM> at the <NUM>° engagement position. From there, continued rotation of the transmission element <NUM> in the first rotary direction will drive rotation of the piston rod driver <NUM> through <NUM>° until the injector element <NUM> reaches the final position again.

By choosing the thread <NUM> of the piston rod <NUM> to have a suitable pitch, it can be arranged that rotation of the piston rod driver <NUM> through <NUM>° results in delivery of the desired fixed dose from the cartridge <NUM>. Rotation of the transmission element <NUM> and the piston rod driver <NUM> through <NUM>° corresponds to movement of the injector element <NUM> through a predetermined axial distance to the position shown in <FIG>. The predetermined distance depends on the pitch of the helical coupling <NUM>,<NUM>. The injector element <NUM> is preferably constrained to prevent it moving through an axial distance significantly greater than the predetermined distance.

Injector pens according to preferred embodiments of the invention also include a last dose lock-out feature, which prevents the pen being used to set and deliver a dose if the drug cartridge <NUM> contains less than the predetermined fixed dose. As the pen is repeatedly re-used, the piston rod <NUM> advances along the axis <NUM> to displace doses of the drug from the cartridge <NUM>. The position of the piston rod <NUM> is thus a measure of the quantity of drug remaining in the cartridge <NUM>.

The piston rod <NUM> comprises an enlarged head <NUM> at its proximal end. <FIG> and <FIG> show the pen at an early stage, when the piston rod <NUM> is at or near its starting position and fully extended in the proximal direction. With the delivery of multiple doses, the piston rod <NUM> advances in the distal direction until the head <NUM> of the piston rod <NUM> approaches the last dose lock-out mechanism shown in <FIG>. The last dose lock-out mechanism comprises a collar <NUM> and a locking element <NUM> that surround the piston rod <NUM> within the transmission element <NUM>. The collar <NUM> is fixed to the second circular ratchet <NUM>. The locking element <NUM> can move axially relative to the collar <NUM> and is urged to move in the first axial direction by a spring <NUM> mounted between the collar <NUM> and the locking element <NUM>. However, the locking element <NUM> is prevented from moving by a pair of hooks <NUM> that engage a proximal surface <NUM> of the collar <NUM>.

As the piston rod <NUM> advances, its head <NUM> comes into contact with inclined cam surfaces <NUM> of the hooks <NUM>. Continued advancement of the head <NUM> in the first axial direction forces the hooks <NUM> apart, as shown by the upper pair of black arrows in <FIG>, until the hooks <NUM> disengage from the proximal surface <NUM>. Now the spring <NUM> drives the locking element <NUM> to move in the first axial direction, as shown by the lower black arrow in <FIG>, so that locking wedges <NUM> are pushed into positions immediately radially inwards from the ratchet arms <NUM> of the second circular ratchet <NUM>. It is recalled that, because the piston rod <NUM> is advancing, the second circular ratchet <NUM> must be engaged. Its ratchet arms <NUM> are therefore flexed outwards to enter the grooves <NUM> of the transmission element <NUM>, leaving spaces behind them for insertion of the wedges <NUM>.

When a subsequent attempt is made to withdraw the injector element <NUM> and rotate the transmission element <NUM> in the second rotary direction to set a new dose, the wedges <NUM> prevent the ratchet arms <NUM> disengaging from the grooves <NUM> so the transmission element <NUM> remains rotationally locked to the piston rod driver <NUM>. The first circular ratchet <NUM> prevents rotation of the piston rod driver <NUM> in the second rotary direction. In turn this prevents rotation of the transmission element <NUM> to set a new dose so the pen becomes inoperable.

As will be apparent, the length of the piston rod <NUM> should be chosen so that its head <NUM> activates the last dose lock-out mechanism on or near completion of delivery of the final full dose in the cartridge <NUM>.

In the preferred embodiment of the invention there are two engagement positions of the second circular ratchet <NUM>, spaced <NUM> degrees apart. However, in alternative embodiments there could be a different number n of engagement positions, equally spaced at <NUM>/n degrees apart. Preferably <NUM> ≤ n ≤ <NUM>.

An alternative configuration of the piston rod driver <NUM> and the piston rod guide <NUM> is possible, which comprises a threaded coupling between the piston rod guide <NUM> and the piston rod <NUM>, and a sliding, co-rotating coupling between the piston rod <NUM> and the piston rod driver <NUM>. For example, the piston rod driver <NUM> may comprise a pair of internal flats to engage the external flats <NUM> of the piston rod <NUM> and force the piston rod <NUM> to co-rotate with the piston rod driver <NUM>. Thereby, rotation of the piston rod <NUM> within the threaded coupling of the piston rod guide <NUM> drives the piston rod <NUM> to advance along the axis <NUM>.

Claim 1:
An injector pen, comprising:
a piston rod guide (<NUM>);
a piston rod driver (<NUM>) configured to rotate without axial movement relative to the piston rod guide (<NUM>);
a piston rod (<NUM>) coupled to the piston rod driver (<NUM>) and to the piston rod guide (<NUM>) such that rotation of the piston rod driver (<NUM>) drives axial movement of the piston rod (<NUM>) relative to the piston rod guide (<NUM>); and
a first circular ratchet (<NUM>) that constrains the piston rod driver (<NUM>) to rotate only in a first rotary direction, which drives the piston rod (<NUM>) to move in a first axial direction;
characterized in that the injector pen further comprises:
a transmission element (<NUM>) configured to rotate without axial movement, relative to the piston rod guide (<NUM>), in the first rotary direction or in a second, opposite rotary direction; and
a second circular ratchet (<NUM>) that selectively engages the transmission element (<NUM>) with the piston rod driver (<NUM>) such that:
rotation of the transmission element (<NUM>) in the second rotary direction is not transmitted to the piston rod driver (<NUM>); and
rotation of the transmission element (<NUM>) in the first rotary direction is transmitted to the piston rod driver (<NUM>) only if the transmission element (<NUM>) has previously been rotated in the second rotary direction through a predetermined angle that is sufficient to permit engagement of the second circular ratchet (<NUM>).